clonebundles: document REQUIREDRAM key
This is a follow-up to 72feaeb510b3, which introduced the feature. The
key should be documented as part of the format specification inside the
extension docstring.
Differential Revision: https://phab.mercurial-scm.org/D8838
# Files that just need to be migrated to the formatter.
# Do not add new files here!
mercurial/cext/manifest.c
mercurial/cext/osutil.c
# Vendored code that we should never format:
syntax: glob
contrib/python-zstandard/**.c
contrib/python-zstandard/**.h
hgext/fsmonitor/pywatchman/**.c
mercurial/thirdparty/**.c
mercurial/thirdparty/**.h