Document cond_neg, cond_one_cmpl, cond_len_neg and cond_len_one_cmpl standard patterns

When I added `cond_one_cmpl` (and the corresponding IFN) I had noticed cond_neg
standard named pattern was not documented and this adds the documentation for
all 4 named patterns now.

OK? Tested by building the manual.

gcc/ChangeLog:

	* doc/md.texi (Standard patterns): Document cond_neg, cond_one_cmpl,
	cond_len_neg and cond_len_one_cmpl.