Minor: comments that explain the schema used in simply_expressions

[alamb]

Which issue does this PR close?

Related to #7699

Rationale for this change

I am somewhat embarrassed that I did not fully understand @Dandandan 's fix in https://github.com/apache/arrow-datafusion/pull/7699/files and then spent some non trivial time tracking the same issue down in IOx and filed a ticket #7746 only to find it had already been fixed

What changes are included in this PR?

Add some comments to try and explain the change more generally

BTW I made a diagram explaining what is going on, that I wanted to share even though I am not sure it belongs in the comments directly:

 Query:                                                                                                                 
 SELECT t.a FROM t WHERE t.b > 5                                                                                        
                                                                                                                        
                                                                                                                        
                                                                                                                        
┌────────────────────┐                                                                                                  
│  Projection(t.a)   │                                                                                                  
└────────────────────┘                                                                                                  
           ▲                                                                                                            
           │                                                                                                            
           │                                                                                                            
┌────────────────────┐        Filter           ┌────────────────────┐     Projection     ┌────────────────────┐         
│  Filter(t.b > 5)   │   ────Pushdown──▶       │  Projection(t.a)   │  ────Pushdown──▶   │  Projection(t.a)   │         
└────────────────────┘                         └────────────────────┘                    └────────────────────┘         
           ▲                                              ▲                                         ▲                   
           │                                              │                                         │                   
           │                                              │                                         │                   
┌────────────────────┐                         ┌────────────────────┐                    ┌────────────────────┐         
│        Scan        │                         │        Scan        │                    │        Scan        │         
└────────────────────┘                         │  filter=(t.b > 5)  │                    │  filter=(t.b > 5)  │         
                                               └────────────────────┘                    │  projection=(t.a)  │         
                                                                                         └────────────────────┘         
                                                                                                                        
Initial Plan                                                                           Projection pushdown notes that   
                                           If `TableProviderFilterPushDown`            the scan only needs t.a          
                                           returns true, filter pushdown                                                
                                           pushes the filter into the scan             BUT internally evaluating the    
                                                                                       predicate still requires t.b     
                                                                                                                        

Are these changes tested?

Are there any user-facing changes?

[comphead]

The diagram is awesome, surely we can find a place for it in docs

[alamb]

The diagram is awesome, surely we can find a place for it in docs

I have included it in #7759

[alamb]

@Dandandan or @comphead could I ask one of you to approve this PR (it needs a review prior to being able to merge):

[comphead]

lgtm

[alamb]

Thank you @comphead