Improve watchman docs

[sandstrom]

Only a few changes (the current text is good):

• Replaced 'may need' with 'we recommend'
• Updated synced 'missing watchman' message, to make this page show up in google results (https://github.com/chrmod/watch-detector/blob/master/lib/index.js#L256)

There are a few more things mentioned in the old docs, but I'm not sure if they're worth moving over, they seem like pretty auxiliary things. If included, it would be neat to fold them into a details/summary block (if the docs site support that).

<details><summary>Debugging & Advanced</summary><!-- more details here --></details>

[jenweber]

Thank you! 🏆 for the first PR.

I don't think we have a summary block, but we could add them at the bottom for now.