docs: minor cosmetic fixes

docs: minor cosmetic fixes

* doc/automake.texi: Break few overly long lines, throughout the
file.
("Simple Tests"): Move @vindex for XFAIL_TESTS to the correct
position, i.e., before and not after the paragraph where it is
introduced.
("Options" @item ansi2knr): Use @pxref instead of @xref.  This
fixes a texinfo warning.
("Other things Automake recognizes" @item AM_C_PROTOTYPES): Use
@pxref instead of @ref.