[patch] Clarify doxygen comments w.r.t nulls in std::string

[Jonathan Wakely]

A stackoverflow poster was confused by the comments on
basic_string::length() which talk about null-temrination, as it isn't
obvious that embedded nulls are not terminators.

I think this makes it clearer, but would appreciate other opinions.

(The @{ and @} commands tell doxygen to use the same comment for both
functions, to avoid duplicating the text more than already necessary
for the two string implementations.)


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