bpo-17232: Clarify docs for -O and -OO command line options (GH-5839)

The 'optimization' is for space in the executable file, not for run time.
(cherry picked from commit 186b606d8a)

Co-authored-by: Cheryl Sabella <cheryl.sabella@gmail.com>

Modules/main.c

@@ -96,8 +96,10 @@ static const char usage_2[] = "\
          if stdin does not appear to be a terminal; also PYTHONINSPECT=x\n\
 -I     : isolate Python from the user's environment (implies -E and -s)\n\
 -m mod : run library module as a script (terminates option list)\n\
--O     : optimize generated bytecode slightly; also PYTHONOPTIMIZE=x\n\
--OO    : remove doc-strings in addition to the -O optimizations\n\
+-O     : remove assert and __debug__-dependent statements; add .opt-1 before\n\
+         .pyc extension; also PYTHONOPTIMIZE=x\n\
+-OO    : do -O changes and also discard docstrings; add .opt-2 before\n\
+         .pyc extension\n\
 -q     : don't print version and copyright messages on interactive startup\n\
 -s     : don't add user site directory to sys.path; also PYTHONNOUSERSITE\n\
 -S     : don't imply 'import site' on initialization\n\