Improve grammar in doc/flatpak-manifest.xml

[jwakely]

I'm not sure what the sentence "Extension define extension points in the app/runtime that can be implemented by extensions, supplying extra files which are available during runtime" is really trying to say (it uses the word extension too many times!) but I think this is better.

I'm also not sure exactly what the no-debuginfo description is saying. Please check my changes make sense.

[TingPing]

I'm not sure what the sentence "Extension define extension points in the app/runtime that can be implemented by extensions, supplying extra files which are available during runtime" is really trying to say (it uses the word extension too many times!) but I think this is better.

The first "Extension" is referring to the item type itself. You could probably omit it and say:

Defines extension points ...

[jwakely]

You could probably omit it and say:

Yes, the same thought had occurred to me. I'll push an update to the branch.