[DOCS] Clarify backport policy for important technical corrections. (#49131)

* [DOCS] Clarify backport policy for important technical corrections.

* Update docs/README.asciidoc

Co-Authored-By: James Rodewig <[email protected]>

Author: debadair

docs/README.asciidoc

@@ -3,6 +3,23 @@ Elasticsearch documentation build process.
 
 See: https://github.com/elastic/docs
 
+=== Backporting doc fixes
+
+* Doc changes should generally be made against master and backported through to the current version
+  (as applicable).  
+  
+* Changes can also be backported to the maintenance version of the previous major version. 
+  This is typically reserved for technical corrections, as it can require resolving more complex
+  merge conflicts, fixing test failures, and figuring out where to apply the change.
+
+* Avoid backporting to out-of-maintenance versions. 
+  Docs follow the same policy as code and fixes are not ordinarily merged to
+  versions that are out of maintenance.
+  
+* Do not backport doc changes to https://www.elastic.co/support/eol[EOL versions]. 
+
+=== Snippet testing
+
 Snippets marked with `[source,console]` are automatically annotated with
 "VIEW IN CONSOLE" and "COPY AS CURL" in the documentation and are automatically
 tested by the command `./gradlew -pdocs check`. To test just the docs from a