Simplify docstring generation in astropy.wcs

This is an esoteric one, and probably doesn't require too much scrutiny beyond Windows experts (@embray).

The docstrings for `_wcs.so` are compiled into the shared object, rather than on-the-side in Python.  The problem has long been that Microsoft Visual Studio doesn't allow very long string literals -- I believe the maximum was something around 255 characters).  So it used to create short strings and then concat them together, which obviously doubled the memory requirement.

This PR uses a technique I just learned about that is reportedly portable, which is to just use character arrays (which reportedly have no limit) rather than string literals.  I've pushed this to my staging branch to give it some testing on Windows.