[Doc] Add documentation for masks in tensor specs

[kurtamohler]

Description

Add documentation for masks in the four *DiscreteTensorSpecs.

Motivation and Context

close #2280

• Someone has raised an issue to propose this change (required for new features and bug fixes)

Types of changes

What types of changes does your code introduce? Remove all that do not apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds core functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)
• Example (update in the folder of examples)

Checklist

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If you are unsure about any of these, don't hesitate to ask. We are here to help!

• I have read the CONTRIBUTION guide (required)
• My change requires a change to the documentation.
• I have updated the tests accordingly (required for a bug fix or a new feature).
• I have updated the documentation accordingly.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/rl/2289

• 📄 Preview Python docs built from this PR

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❗ 1 Active SEVs

There are 1 currently active SEVs. If your PR is affected, please view them below:

• Too many GPU instances requested - any *.nvidia.gpu and ephemeral instances should experience significant queue times

❌ 2 New Failures, 4 Pending, 16 Unrelated Failures

As of commit b0cc6f5 with merge base 8e43ac8 ():

NEW FAILURES - The following jobs have failed:

• Continuous Benchmark (PR) / CPU Pytest benchmark (gh)
  Workflow failed! Resource not accessible by integration
• Continuous Benchmark (PR) / GPU Pytest benchmark (gh)
  Workflow failed! Resource not accessible by integration

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• Examples Tests on Linux / tests (3.9, 12.1) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128
• Generate documentation / build-docs (3.9, 12.1) / linux-job (gh) (similar failure)
• Habitat Tests on Linux / tests (3.9, 12.1) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128
• Libs Tests on Linux / unittests-gym (3.9, 12.1) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• Libs Tests on Linux / unittests-sklearn (3.9, 12.1) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128
• Lint / c-source / linux-job (gh) (similar failure)
• Lint / python-source-and-configs / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128
• RLHF Tests on Linux / unittests (3.9, 12.1) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• Unit-tests on Linux / tests-cpu (3.10) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• Unit-tests on Linux / tests-cpu (3.11) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128
• Unit-tests on Linux / tests-cpu (3.8) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• Unit-tests on Linux / tests-cpu (3.9) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128
• Unit-tests on Linux / tests-gpu (3.10, 12.1) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128
• Unit-tests on Linux / tests-olddeps (3.8, 11.6) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128
• Unit-tests on Linux / tests-optdeps (3.10, 12.1) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128
• Unit-tests on Linux / tests-stable-gpu (3.10, 11.8) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128

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[vmoens]

LGTM thanks for this!