ExternalProject: Clarify SVN_REVISION format in docs (#15081)

Show the explicit '-r' that is required.
FindOpenMP: Add support for Fortran (#14656)
Merge topic 'FindOpenMP-add-Fortran-support' into next

d0678408 FindOpenMP: Add support for Fortran (#14656)
Merge topic 'ExternalProject-doc-SVN_REVISION-format' into next

b26faaf1 ExternalProject: Clarify SVN_REVISION format in docs (#15081)