'make docs' returns 0 even on error, could use a more restrictive "find" expression

The "docs" target of Hue's root Makefile will return "0" even if the docs submake compilation fails. This causes build dependencies which require the "docs" target to succeed even if the docs were not in fact successfully built.

Additionally, the "docs" target tries to find all files of the form ".txt". This causes issues if you have one of the docs files open in vim, because it will create a .swp..txt file which then won't compile.