[analyzer][docs] Ellaborate the docs of cplusplus.StringChecker

Let's describe accurately what the users can expect from the checker in
a direct way.
Also, add an example warning message.

Reviewed By: martong, Szelethus

Differential Revision: https://reviews.llvm.org/D113401

GitOrigin-RevId: bf55b9f0d0e938def5d24629325b271cbfc3b04a
1 file changed