Docs: add docstrings for schematics/utils

[rjlopezdev]

I'm submitting a...

[ ] Regression
[ ] Bug report
[ ] Feature request
[x] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead post your question on Stack Overflow.

Current behavior

Currently there aren't docs for this methods.

Expected behavior

Add docstrings to improve schematics development experience.
Ex. VSCode feedback with docstring when hover on method.

Minimal reproduction of the problem with instructions

N/A

What is the motivation / use case for changing the behavior?

Schematics are becoming so popular. Developers should be notified that Nest contains with some utility functions that can be used for this purpose :)

Environment

Nest version: 6.14.2

 
For Tooling issues:
- Node version:  10.16.3
- Platform:  Mac OS Catalina

[KingDarBoja]

This is something that is needed on Angular Schematics as well, so developers can construct their own schematics and contribute to either the project or for its own use. A clear example would be a migration schematic based on migration guide docs.

[kamilmysliwiec]

#1945