Improve documentation

[AndyGaskell]

I think we could improve the documentation.

Some parts of https://php-fcm.readthedocs.io/ are unclear or out of date.

Just jotting this down as an issue so I remember to come back to it.

If anyone has any thoughts on documentation, please jot them here, like...

• Bits that are unclear
• Bits that are out of date
• Features or options that are not documented
• Any other ideas to improve the docs