doc: revise deprecation semverness info in Collaborator Guide

Simplify and clarify deprecation semverness information in the
Collaborator Guide. Unlike some of the other changes I've made lately,
this one is not merely cosmetic. It changes information about how to
handle deprecations vis-a-vis SemVer. The revised conventions take
advange of `notable change` labels etc. instead of suggesting that
doc-deprecations be treated as `semver-minor`. The idea that a
deprecation is a new feature seems incorrect from a SemVer perspective,
but probably made sense at the time the text was written if we weren't
yet using `notable change` etc.

PR-URL: #26232
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

COLLABORATOR_GUIDE.md

@@ -339,13 +339,13 @@ documentation must state the deprecation status.
   * Backward-incompatible changes including complete removal of such APIs may
     occur at any time.
 
-Documentation-Only Deprecations may be handled as semver-minor or semver-major
-changes. Such deprecations have no impact on the successful operation of running
-code and therefore should not be viewed as breaking changes.
+Apply the `notable change` label to all pull requests that introduce
+Documentation-Only Deprecations. Such deprecations have no impact on code
+execution. Thus, they are not breaking changes (`semver-major`).
 
-Runtime Deprecations and End-of-life APIs (internal or public) must be
-handled as semver-major changes unless there is TSC consensus to land the
-deprecation as a semver-minor.
+Runtime Deprecations and End-of-life APIs (internal or public) are breaking
+changes (`semver-major`). The TSC may make exceptions, deciding that one of
+these deprecations is not a breaking change.
 
 All Documentation-Only and Runtime deprecations will be assigned a unique
 identifier that can be used to persistently refer to the deprecation in