[Docs] Schema class docs, Kind glossary entry, mention of Private _Apply()

[rlei-weta]

USD/pxr/usd/lib/usd is outdated:

• https://graphics.pixar.com/usd/docs/Generating-New-Schema-Classes.html#GeneratingNewSchemaClasses-WhatisaSchemaClass?

Mention of removed private _Apply():

• https://graphics.pixar.com/usd/docs/Generating-New-Schema-Classes.html#GeneratingNewSchemaClasses-TypesofSchemaClasses
• https://graphics.pixar.com/usd/docs/api/_usd__page__generating_schemas.html

"pusblished":

• https://graphics.pixar.com/usd/docs/USD-Glossary.html#USDGlossary-Kind

This next one is more of a suggestion - in the Note section of https://graphics.pixar.com/usd/docs/api/_usd__page__generating_schemas.html#Usd_SchemaCodeGeneration, moduleDeps.cpp also deserves a mention that it is not auto-genned nor updated.  At first glance it may seem like a boilerplate file due to its presence in all of the schema libraries.  

Furthermore, moduleDeps.cpp also deserves a separate mention that it is meant to be built as part of the C++ library as opposed to the python module. We made this mistake before understanding the purpose of it - as did a another facility. It was fairly subtle until we hit a particular use case :smile:

An excerpt about its exact contribution to the runtime would go wonders!

[jilliene]

Filed as internal issue #USD-6464

[spiffmon]

I've addressed the non-doxygen related issues, and pushed out the results. The section in the schema generation tutorial was much more grievously out of date than you noted, and I think I've patched it all up.

Thanks, @rlei-weta , I'll leave this open for the doxygen issues.