[Internal]: Add clarification for updating integration docs

[jmikell821]

We should clarify/add a few things to the how-to guide that specifies how to update our integration docs:

• We need to clarify this part: Edit the source file at packages/{package}/docs/README.md or packages/{package}/_dev/build/docs/README.md. Contributors should preferably edit the source file at packages/{package}/_dev/build/docs/README.md so that the former file doesn't get overwritten the next time integrations are built. It's OK to edit the packages/{package}/docs/README.md file
• We should add a note about what to do when there are subsequent edits to the PR, that the contributor needs to re-run the elastic-package build command (but not the full commands that bump the version up).