docs: add --if-present to run-script docs (#20279)

PR-URL: npm/npm#20279
Credit: @aleclarson
Reviewed-By: @iarna

doc/cli/npm-run-script.md

@@ -68,6 +68,10 @@ you will be given a warning to run `npm install`, just in case you've forgotten.
 
 You can use the `--silent` flag to prevent showing `npm ERR!` output on error.
 
+You can use the `--if-present` flag to avoid exiting with a non-zero exit code
+when the script is undefined. This lets you run potentially undefined scripts
+without breaking the execution chain.
+
 ## SEE ALSO
 
 * npm-scripts(7)