Documentation tweaks for internal-fn-related optabs

Documentation tweaks for internal-fn-related optabs

As Bernd requested, this patch adds "This pattern cannot FAIL" to the
documentation of optabs that came to be mapped to interal functions.
For consistency I did the same for optabs that were already being
used for internal functions.

Many of the optabs weren't documented in the first place, so I added
entries for the missing ones.  Also, there were some inaccuracies in
the documentation of the rounding optabs.  The bitcount optabs said
that operand 0 has mode @var{m} and that operand 1 is under target
control, whereas it should be the other way around.

Tested on x86_64-linux-gnu.

gcc/
	* doc/md.texi (vec_load_lanes@var{m}@var{n}): Document that
	the pattern cannot FAIL.
	(vec_store_lanes@var{m}@var{n}): Likewise.
	(maskload@var{m}@var{n}): Likewise.
	(maskstore@var{m}@var{n}): Likewise.  Fix a cut-&-paste error
	in the name of the pattern.
	(rsqrt@var{m}2): Document that mode m must be a scalar or vector
	floating-point mode and that all operands have that mode.
	(fmin@var{m}3, fmax@var{m}3): Likewise.  Document that the
	pattern cannot FAIL.
	(sqrt@var{m}2): Document that mode m must be a scalar or vector
	floating-point mode, that all operands have that mode, and that
	the patterns cannot FAIL.  Remove previous documentation referring
	to @code{double} and @code{float}.
	(fmod@var{m}3, remainder@var{m}3, cos@var{m}2, sin@var{m}2)
	(sincos@var{m}3, log@var{m}2, pow@var{m}3, atan2@var{m}3)
	(copysign@var{m}3): Likewise.
	(exp@var{m}2): Likewise.  Explicitly state the base.
	(floor@var{m}2): As for sqrt@var{m}2, but also specify the operands.
	(btrunc@var{m}2, rint@var{m}2): Likewise.
	(round@var{m}2): Likewise.  Fix incorrect description of rounding
	effect.
	(ceil@var{m}2): As for round@var{m}2.
	(nearbyint@var{m}2): As for floor@var{m}2, but also mention that
	the instruction must not raise an inexact condition.
	(scalb@var{m}3): Document previously-undocumented pattern
	(ldexp@var{m}3, tan@var{m}2, asin@var{m}2, acos@var{m}2)
	(atan@var{m}2, expm1@var{m}2, exp10@var{m}2, exp2@var{m}2)
	(log1p@var{m}2, log10@var{m}2, log2@var{m}2, logb@var{m}2)
	(significand@var{m}2): Likewise.
	(ffs@var{m}2): Fix the description of the modes, so that operand 1 has
	mode m and operand 0 is defined more freely.  Document that @var{m}
	can be a scalar or vector integer mode and that the pattern is not
	allowed to FAIL.
	(clz@var{m}2, ctz@var{m}2, popcount@var{m}2, parity@var{m}2): Likewise.
	(clrsb@var{m}2): Likewise, except that the description of the
	mode was missing in this case.

From-SVN: r231230