Require documentation for every item of the API reference

The DocFX gate only failed on a broken link or an unresolved reference.
DocFX does not report a declaration without a comment, and the API had
none, so the gate passed over a reference with no descriptions at all.

The gate now has three parts:

- DocFX still builds the reference with warnings as errors.
- The API project compiles its XML documentation. With warnings as errors
  already on, an undocumented public declaration or a wrong cref now fails
  the build.
- docs/api/CheckDocumentation.cs reads the metadata DocFX generated and
  fails when a namespace, type or member has no summary. This covers
  internal and private members, which the compiler does not check. It runs
  as a file-based .NET program, so the gate needs no tool beyond the SDK.
  It refuses to pass when the metadata is missing or defines nothing.

To satisfy the gate, every declaration of apps/api has documentation
comments, 131 reference items in all, and the two namespaces are described
by overwrite files in docs/api/namespaces. Only comment lines were added to
the C# sources; no statement changed.

docs/api/filterConfig.yml leaves generator-written code out of the
reference: the generated regular-expression namespace, and the one partial
method whose generated half replaces the comment of its declaration.

Verified locally: the solution builds with no warnings, the 23 API tests
pass, DocFX builds with no warnings and the coverage check passes. Removing
one summary and one namespace file made the check fail and name both.

0 files changed, 0 insertions, 0 deletions