Clarify thread-safety comment for FlagSaver.

Tested:
blaze test base:commandlineflags_unittest

R=csilvers
DELTA=4  (3 added, 0 deleted, 1 changed)


Revision created by MOE tool push_codebase.
MOE_MIGRATION=3791


git-svn-id: https://gflags.googlecode.com/svn/trunk@71 6586e3c6-dcc4-952a-343f-ff74eb82781d