Add falsy regexes, primarily for session functions

[timsmid]

This PR adds three additional regexes to the falsies detection list:

• '/&false;\s+for\s+failure/m' catches some session functions and coincidentally also a mbstring function.
• '/On failure to change the value, &false; is returned./m' specifically catches session_cache_expire and session_cache_limiter.
• '/This function returns &true; if a session was successfully started,\s+otherwise &false;./m' specifically catches session_start.

Method::stripReturnFalseText has been modified to strip the descriptions where necessary.

[shish]

Looks good, thank you <3