gdb/doc: add section about Fortran intrinsic functions and types

The earlier version of this document had no sections about the
available Fortran intrinsic functions or the Fortran builtin types.

I added two sections 'Fortran intrinsics' and 'Fortran types' to
document the available Fortran language features.  The subsection
'Fortran Defaults' has been integrated into the Fortran subsection.