mmapindex: move the 'mmapindexthreshold' option out of experimental
The option is useful and should be advertised more. We move it out of
experimental as a first step. The storage section is selected as this is
related to how the storage is accessed. A new 'performance' section might be
We move from 'mmapindexthreshold` to mmap-threshold as non-index item are
also suitable for mmap (eg: the rev-branch-cache).
If relevant, we can introduce sub-option mmap-threshold.revlog-index later.