Update NEWS feature test macro description of TS 18661-1 support.

Update NEWS feature test macro description of TS 18661-1 support.

The NEWS entry for the feature test macro
__STDC_WANT_IEC_60559_BFP_EXT__ describes the state of support for
that TS as "most features from that TS are not supported by the GNU C
Library".  This patch updates it to say "not all features from that TS
are supported by the GNU C Library".

(The functions not yet supported are the fromfp functions - I'm
working on them, but they may not be done before the freeze - and the
functions round result to narrower type - which definitely won't be
started for 2.25, since they require significant infrastructure work.
That's 30 functions, which is less than half the number of functions
in the TS, so saying "most" now seems inaccurate.)