Clean up documentation for -Wstrict-flex-arrays [PR111659]

Clean up documentation for -Wstrict-flex-arrays [PR111659]

gcc/ChangeLog
	PR middle-end/111659
	* doc/extend.texi (Common Variable Attributes): Fix long lines
	in documentation of strict_flex_array + other minor copy-editing.
	Add a cross-reference to -Wstrict-flex-arrays.
	* doc/invoke.texi (Option Summary): Fix whitespace in tables
	before -fstrict-flex-arrays and -Wstrict-flex-arrays.
	(C Dialect Options): Combine the docs for the two
	-fstrict-flex-arrays forms into a single entry.  Note this option
	is for C/C++ only.  Add a cross-reference to -Wstrict-flex-arrays.
	(Warning Options): Note -Wstrict-flex-arrays is for C/C++ only.
	Minor copy-editing.  Add cross references to the strict_flex_array
	attribute and -fstrict-flex-arrays option.  Add note that this
	option depends on -ftree-vrp.