doc: Remove documentation of Mac OS X <= 10.4 as a supported platform.
* doc/gnulib-intro.texi (Target Platforms): Mention that Mac OS X 10.4
and older is unsupported.
* doc/posix-functions/acosl.texi: Don't mention workarounds specific to
Mac OS X 10.4 and older.
* doc/posix-functions/asinl.texi: Likewise.
* doc/posix-functions/atanl.texi: Likewise.
* doc/posix-functions/cosl.texi: Likewise.
* doc/posix-functions/expl.texi: Likewise.
* doc/posix-functions/frexpl.texi: Likewise.
* doc/posix-functions/gettimeofday.texi: Likewise.
* doc/posix-functions/logl.texi: Likewise.
* doc/posix-functions/mkstemp.texi: Likewise.
* doc/posix-functions/sinl.texi: Likewise.
* doc/posix-functions/sqrtl.texi: Likewise.
* doc/posix-functions/tanl.texi: Likewise.
* doc/posix-functions/wcswidth.texi: Likewise.
* doc/**/*.texi: Update.