add docs on CI workflow inputs

[jameslamb]

Description

Contributes to rapidsai/shared-workflows#376

• adds descriptions for all inputs to workflows triggered by workflow_dispatch

Contributes to rapidsai/shared-workflows#379

• removes input enable_check_symbols for conda-cpp-checks workflow... no longer needed as of conda-cpp-post-build-checks: switch to always running by default, add docs shared-workflows#382

Notes for Reviewers

Motivation

The input descriptions show up in the UI when you go to trigger these workflows. Like this:

I'm hoping that will make it easier for developers to manually trigger workflows. Inspired by being asked multiple times "what format is date supposed to be in?".

Removing enable_check_symbols here is a step towards completely removing that always-true input, as part of rapidsai/shared-workflows#379

Other repos?

This is the first of these PRs... let's use it to decide on phrasing that we like.

Once it's approved and merged, I'll open similar PRs across RAPIDS.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[jameslamb]

/merge