docs: Attempt to clarify complex literal suffixes [PR112960]

docs: Attempt to clarify complex literal suffixes [PR112960]

This attempts to clarify Complex literal suffixes in the documentation.

2025-03-07  Jakub Jelinek  <jakub@redhat.com>

	PR c/112960
	PR c/117029
	* doc/extend.texi (Complex): Add I and J suffixes to the list of
	complex suffixes, adjust for all of those being part of ISO C2Y,
	clarify that for -fno-ext-numeric-literals none of those are
	recognized as GNU extensions and for C++14 i is considered UDL
	even for -fext-numeric-literals when <complex> is included.