Add Documentation for Execution Results Handling in Clang-Repl (#65650)

[clang-repl] Add Documentation for Execution Results Handling.

This patch adds documentation for execution results handling in
Clang-REPL with
the below features:

- Automatic Printf feature
- Value Synthesis feature
- Pretty Printing feature


Continuing this work https://reviews.llvm.org/D156858 with this PR. I am
issuing this patch on behalf of Saqib.

Reviewers:
@vgvassilev 
@junaire