Add example for select.join

[sypets]

• Add small example for select.join.
• Improve explanatory text

Previously, the explanatory text was slightly misleading because it instructed to use the table name where in fact an entire join clause (without the word "JOIN") can be used including aliasing the table and adding a constraint with "ON".

[brotkrueml]

@sypets Thanks for the improvement 👍

[github-actions]

The backport to 11.5 failed:

The process '/usr/bin/git' failed with exit code 1

stderr

error: could not apply 2fd2502... Add example for select.join
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".

stdout

Auto-merging Documentation/Functions/Select.rst
CONFLICT (content): Merge conflict in Documentation/Functions/Select.rst

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-11.5 11.5
# Navigate to the new working tree
cd .worktrees/backport-11.5
# Create a new branch
git switch --create backport-775-to-11.5
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 2fd2502662ad51ef98e79870bbb1721d5bd2d457,2c1691d63b803f42f9fe16a5051a4c2d11c413dd
# Push it to GitHub
git push --set-upstream origin backport-775-to-11.5
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-11.5

Then, create a pull request where the base branch is 11.5 and the compare/head branch is backport-775-to-11.5.