Add documentation and release policy on "latest" constraints (#21093)

(cherry picked from commit aac5a1d)

dev/README_RELEASE_AIRFLOW.md

@@ -156,7 +156,7 @@ For now this is done manually, example run  `git log --oneline v2-2-test..HEAD -
     popd
     ```
 
-- Tag & Push the latest constraints files. This pushes constraints with rc suffix (this is expected)!
+- Tag & Push the constraints files. This pushes constraints with rc suffix (this is expected)!
 
     ```shell script
     git checkout origin/constraints-${VERSION_BRANCH}
@@ -298,6 +298,7 @@ protected_branches:
    git push origin tag constraints-${BRANCH_PREFIX}
    ```
 
+
 ## Prepare PyPI convenience "snapshot" packages
 
 At this point we have the artifact that we vote on, but as a convenience to developers we also want to
@@ -796,6 +797,13 @@ At this point we release an official package:
     git push origin tag "constraints-${VERSION}"
     ```
 
+- In case you release "latest stable" version, also update "latest" constraints
+
+    ```shell script
+    git tag -f -s "constraints-latest" -m "Latest constraints set to Apache Airflow ${VERSION}"
+    git push origin tag "constraints-latest"
+    ```
+
 - Push Tag for the final version
 
     This step should only be done now and not before, because it triggers an automated build of

docs/apache-airflow/installation/installing-from-pypi.rst

@@ -81,6 +81,13 @@ You can create the URL to the file substituting the variables in the template be
 
   https://raw.githubusercontent.com/apache/airflow/constraints-${AIRFLOW_VERSION}/constraints-no-providers-${PYTHON_VERSION}.txt
 
+You can also use "latest" as version when you install "latest" stable version of Airflow. The "latest"
+constraints always points to the "latest" released Airflow version constraints:
+
+.. code-block::
+
+  https://raw.githubusercontent.com/apache/airflow/constraints-latest/constraints-3.7.txt
+
 Installation and upgrade scenarios
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