Document USD_DEFAULT_FILE_FORMAT Usage

[mati-nvidia]

Description of Issue

The USD_DEFAULT_FILE_FORMAT environment variable is currently only documented in the usdcat help: https://openusd.org/release/toolset.html#usdcat and the accepted values are only discoverable by looking at the source code. We should make this easier to discover.

One possible place for this information is here: https://openusd.org/release/usdfaq.html#what-file-format-is-my-usd-file I can create a PR for this issue if we like that solution.

[mati-nvidia]

See linked PR for the proposed improvement. We would like to try to squeeze this in for this upcoming release so that we can link to it from our Omniverse developer docs.

[jesschimein]

Filed as internal issue #USD-9212

[jesschimein]

Closing as completed; see #2924