doc: Add documentation of which operand matches the mode of the standard pattern name [PR113508]

In some of the standard pattern names, it is not obvious which mode is being used in the pattern
name. Is it operand 0, 1, or 2? Is it the wider mode or the narrower mode?
This fixes that so there is no confusion by adding a sentence to some of them.

Built the documentation to make sure that it builds.

gcc/ChangeLog:

	PR middle-end/113508
	* doc/md.texi (sdot_prod@var{m}, udot_prod@var{m},
	usdot_prod@var{m}, ssad@var{m}, usad@var{m}, widen_usum@var{m}3,
	smulhs@var{m}3, umulhs@var{m}3, smulhrs@var{m}3, umulhrs@var{m}3):
	Add sentence about what the mode m is.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>