doc: Fix errors with makeinfo 4.8 [PR122638]

doc: Fix errors with makeinfo 4.8 [PR122638]

When building trunk on Mac OS X 10.13 with the bundled makeinfo 4.8, I
ran into a couple of errors:

gcc/doc/install.texi:2675: @option expected braces.
gcc/doc/install.texi:2675: Misplaced {.
gcc/doc/install.texi:2675: Misplaced }.
gcc/doc/install.texi:2675: @code missing close brace.

gcc/fortran/gfortran.texi:1842: First argument to cross-reference may not be empty.
gcc/fortran/gfortran.texi:1903: First argument to cross-reference may not be empty.
gcc/fortran/intrinsic.texi:15549: Unknown command `cindex,'.

However, install.texi states that makeinfo >= 4.7 is required, so this
should work.

This patch fixes those errors.

Tested on x86_64-apple-darwin17.7.0 (makeinfo 4.8), i386-pc-solaris2.11
(makeinfo 7.2), and x86_64-pc-linux-gnu (makeinfo 7.1).

2025-10-22  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>

	gcc:
	PR other/122638
	* doc/install.texi (Configuration, --enable-x86-64-mfentry): Fix
	typo.

	gcc/fortran:
	PR other/122638
	* gfortran.texi (OpenMP): Fix syntax.
	* intrinsic.texi (UINT): Fix syntax.