Inherit doc return type from class

[P1X3]

Checklist

• Issue number for this PR: Shadowed type generic in doc; Inherit doc type generic from class #2639
• Docs included?: no
• Test units included?: no
• In a clean directory, yarn install, yarn test run successfully? no

Description

Updated doc function generic return type to inherit type from class generic

Code sample

Without this change defining type on collection function call does not pass it to doc function call:
db.collection<Location>('locations').doc('someid').valueChanges(); // Returns Observable<unknown>
db.collection<Location>('locations').doc<Location>('someid').valueChanges(); // Returns Observable<Location>

With this PR calling collection function with type will pass it to doc function call.
db.collection<Location>('locations').doc('someid').valueChanges(); // Returns Observable<Location>
db.collection<Location>('locations').doc<Location>('someid').valueChanges(); // Returns Observable<Location>

[P1X3]

Browsing deeper into other PR I noticed that this PR will will conflict with #2573

[jamesdaniels]

Thanks, appreciate this. Aiming to start prepping for the 6.1 release shortly and will take a look at these PRs.