Documentation for legacysupport.use_mp_libcxx

[aeiouaeiouaeiouaeiouaeiouaeiou]

https://github.com/macports/macports-ports/pull/18690#issuecomment-1547536736


Currently, the LegacySupport documentation only describes how to use the 
library's replacement methods, but not how to install the updated libc++ 
through the portgroup. It would be nice to describe where and how to use 
macports-libcxx, as the number of ports using the latest C++ standards 
continues to grow.


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