The freeze index API docs state that frozen indices are blocked for
write operations.
While this implies frozen indices are read-only, it does not explicitly
use the term "read-only", which is found in other docs, such as the
force merge docs.
This adds the "ready-only" term to the freeze index API docs as well
as other clarification.
This commit adds a rest endpoint for freezing and unfreezing an index.
Among other cleanups mainly fixing an issue accessing package private APIs
from a plugin that got caught by integration tests this change also adds
documentation for frozen indices.
Note: frozen indices are marked as `beta` and available as a basic feature.
Relates to #34352
James Rodewig
Lisa Cawley
Simon Willnauer