Adjust the depth (number of #) for some sections.
Move an underscore prefix to a suffix in an example to let the Fortran
parser in the documentation generator pass.
This document aims to give insights at the representation of parameterized
derived-type (PDTs) in FIR and how PDTs are lowered to FIR and interact
with the runtime.
Reviewed By: jeanPerier, klausler
Differential Revision: https://reviews.llvm.org/D133096