Add default value to each one of the usages of `allow_no_indices`
since it differs between different APIs.
Relates to: #52534
(cherry picked from commit 2eb986488ac326d6da6ab8ad0203a94e08684a36)
Some small clarifications about force-merging and global ordinals, particularly
that global ordinals are cheap on a single-segment index and how this relates
to frozen indices.
Fixes#41687
Several files in the REST APIs nav section are included using
:leveloffset: tags. This increments headings (h2 -> h3, h3 -> h4, etc.)
in those files and removes the :leveloffset: tags.
Other supporting changes:
* Alphabetizes top-level REST API nav items.
* Change 'indices APIs' heading to 'index APIs.'
* Changes 'Snapshot lifecycle management' heading to sentence case.
This adds an API for force merging lucene segments. The `/_optimize` API is now
deprecated and replaced by the `/_forcemerge` API, which has all the same flags
and action, just a different name.
Lisa Cawley
Marios Trivyzas
James Rodewig
David Turner
Frederik Bosch
Mueed Chaudhry
javanna
Matija Bruncic
Nik Everett
Lee Hinman