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

hgext/clonebundles.py

@@ -156,6 +156,14 b' REQUIRESNI'
 
    Value should be "true".
 
+REQUIREDRAM
+   Value specifies expected memory requirements to decode the payload.
+   Values can have suffixes for common bytes sizes. e.g. "64MB".
+
+   This key is often used with zstd-compressed bundles using a high
+   compression level / window size, which can require 100+ MB of memory
+   to decode.
+
 heads
    Used for pull bundles. This contains the ``;`` separated changeset
    hashes of the heads of the bundle content.