VAX/doc: Fix issues with FP format option documentation

VAX/doc: Fix issues with FP format option documentation

Use the correct names of the D_floating and G_floating data formats as
per the VAX ISA nomenclature[1].  Document the `-md', `-md-float', and
`-mg-float' options.

References:

[1] DEC STD 032-0 "VAX Architecture Standard", Digital Equipment
    Corporation, A-DS-EL-00032-00-0 Rev J, December 15, 1989, Section
    1.2 "Data Types", pp. 1-7, 1-9

	gcc/
	* doc/invoke.texi (Option Summary): Add `-md', `-md-float', and
	`-mg-float' options.  Reorder, matching VAX Options.
	(VAX Options): Reword the description of `-mg' option.  Add
	`-md', `-md-float', and `-mg-float' options.