doc: unambiguously mark deprecated signatures

Currently, deprecation notices are always right between two function
signatures and it's virtually impossible to be certain whether they
refer to the previous signature or the next signature.

PR-URL: #942
Reviewed-By: Nicola Del Gobbo <nicoladelgobbo@gmail.com>
Reviewed-By: Gabriel Schulhof <gabrielschulhof@gmail.com>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>

doc/property_descriptor.md

@@ -138,7 +138,7 @@ The name of the property can be any of the following types:
 - `napi_value value`
 - `Napi::Name`
 
-**This signature is deprecated. It will result in a memory leak if used.**
+**The above signature is deprecated. It will result in a memory leak if used.**
 
 ```cpp
 static Napi::PropertyDescriptor Napi::PropertyDescriptor::Accessor (
@@ -186,7 +186,7 @@ The name of the property can be any of the following types:
 - `napi_value value`
 - `Napi::Name`
 
-**This signature is deprecated. It will result in a memory leak if used.**
+**The above signature is deprecated. It will result in a memory leak if used.**
 
 ```cpp
 static Napi::PropertyDescriptor Napi::PropertyDescriptor::Accessor (
@@ -236,7 +236,7 @@ The name of the property can be any of the following types:
 - `napi_value value`
 - `Napi::Name`
 
-**This signature is deprecated. It will result in a memory leak if used.**
+**The above signature is deprecated. It will result in a memory leak if used.**
 
 ```cpp
 static Napi::PropertyDescriptor Napi::PropertyDescriptor::Function (