[arm] Document missing -mfpu entries.

[arm] Document missing -mfpu entries.

Nick pointed out that I hadn't documented the new -mfpu option
neon-vfpv3 and mentioned that some others were missing.

Having looked through the list only one (neon-fp16) really should be
documented; the other two entries in the real table should not be
documented as they are aliases kept for legacy compatibility reasons.
This patch adds the missing entries and notes in the main table that
the other two entries should not be documented.

I've also fixed a small spelling error in the accompanying text.

	* config/tc-arm.c (arm_fpus): Note entires that should not be
	documented.
	* doc/c-arm.texi (-mfpu): Add missing FPU entries for neon-vfpv3 and
	neon-fp16.  Fix spelling error.