Fortran: Use the present tense for the manual.

Fortran: Use the present tense for the manual.

The present tense is preferred for expressing facts or enduring
behavior.  Thus we should say "option X does Y" instead of "option X
will do Y", reserving the future tense for things that happen at some
later time (such as in a future release of GCC, or at run time as
explicitly contrasted with compile time).

This set of edits is largely mechanical substitution of phrasing
involving "will".  I also fixed a few more markup problems noted while
editing nearby text and fixed a few instances of awkward wording.

gcc/fortran/ChangeLog
	* gfortran.texi: Use the present tense throughout; fix some
	markup issues and awkward wording.
	* invoke.texi: Likewise.