docs: Re-write string docs

[max-sixty]

I'm not sure we actually should have done the r strings — we're compiling for another language, not the final result, so \t\t would have been pretty reasonable to keep as \t\t rather than two tab characters.

I'm not in a rush to revert though — folks can always just use the r-strings to get the same thing.

[max-sixty]

Closes #2989

[prql-bot]

The backport to web failed:

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

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-web web
# Navigate to the new working tree
cd .worktrees/backport-web
# Create a new branch
git switch --create backport-3281-to-web
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 f6c4096428b4a427d234574d5212d77a6ae2c123
# Push it to GitHub
git push --set-upstream origin backport-3281-to-web
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-web

Then, create a pull request where the base branch is web and the compare/head branch is backport-3281-to-web.