Add doc describing how (?x) ignores spaces in character classes

FULL PRODUCT VERSION :

A DESCRIPTION OF THE PROBLEM :

Using the regex "[ ]" works, but using "(?x)[ ]" does not. The whitespace is ignored and the result is an error. There's a stackoverflow post about it here: https://stackoverflow.com/questions/49264034/

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Whitespace should be ignored when using the x flag (COMMENTS) but not when it's in a character class

REPRODUCIBILITY :
This bug can be reproduced always.

CUSTOMER SUBMITTED WORKAROUND :
Escaping the space or moving the inline modifier to after the character class works:

x flag with "\\ " or "[\\ ]"
Other option "[ ](?x)