Add back mention of local scoping with let blocks

[amellnik]

This was present in the 0.3 manual but went away in 0.4. Right now it's not mentioned in control flow either, and probably should be added in at some point, but this will involve a more-detailed addition to the docs.

[tkelman]

It's generally preferred to open PR's against master first, they can be backported to the release branch later.

[amellnik]

Whoops, I clicked on the "edit this" button on the doc site and didn't notice the version. Should this go into the 0.4.3 branch or 0.5?

[mauro3]

There is a big scope-manual update in PR #12146. There is a dedicated section on let-blocks, where this sentence could go: https://github.com/mauro3/julia/blob/abdfc24ac1a7acd67b474a1848758589d4613065/doc/manual/variables-and-scoping.rst#for-loops-and-comprehensions

[amellnik]

Sounds good, I'll push there. Edit: actually, it looks like it's well handled there so I'll just close.