libstdc++: Document thread-safety for COW std::string [PR21334]

libstdc++: Document thread-safety for COW std::string [PR21334]

The gcc4-compatible copy-on-write std::string does not conform to the
C++11 requirements on data race avoidance in standard containers.
Specifically, calling non-const member functions such as begin() and
data() needs to do the "copy on write" operation and so is most
definitely a modification of the object. As such, those non-const
members must not be called concurrently with any other uses of the
string object.

libstdc++-v3/ChangeLog:

	PR libstdc++/21334
	* doc/xml/manual/using.xml: Document that container data race
	avoidance rules do not apply to COW std::string.
	* doc/html/*: Regenerate.

(cherry picked from commit dd35f66287b7cca196a720c9641e463255dceb1c)