Expand docstring on SynModuleDecl.Types

[Smaug123]

Description

Expand the ambiguous docstring on SynModuleDecl.Types.

This change is in src/Compiler, but I really don't think it merits a release note.

Correctness: the only construction of this DU case is in the fsyacc parser, and if you can decipher that then you're better than I am. But I did check with the Fantomas tools.

Checklist

• [ ] Test cases added

• [ ] Performance benchmarks added in case of performance changes

• [ ] Release notes entry updated:

  Please make sure to add an entry with short succinct description of the change as well as link to this pull request to the respective release notes file, if applicable.

  Release notes files:

  • If anything under src/Compiler has been changed, please make sure to make an entry in docs/release-notes/.FSharp.Compiler.Service/<version>.md, where <version> is usually "highest" one, e.g. 42.8.200
  • If language feature was added (i.e. LanguageFeatures.fsi was changed), please add it to docs/release-notes/.Language/preview.md
  • If a change to FSharp.Core was made, please make sure to edit docs/release-notes/.FSharp.Core/<version>.md where version is "highest" one, e.g. 8.0.200.

  Information about the release notes entries format can be found in the documentation. Example:

  • More inlines for Result module. (PR #16106)
  • Correctly handle assembly imports with public key token of 0 length. (Issue #16359, PR #16363) *while! (Language suggestion #1038, PR #14238)

  If you believe that release notes are not necessary for this PR, please add NO_RELEASE_NOTES label to the pull request.

[github-actions[bot]]

:warning: Release notes required, but author opted out

[!WARNING] Author opted out of release notes, check is disabled for this pull request. cc @dotnet/fsharp-team-msft