Langref misc fix and improvement (#14695)

* langref: improve the Hello World section

Clarify that a Zig source file must have the .zig extension.

Fix a typo.

* langref: improve the Comments section

  - Document how doc comments are used by -femit-docs
  - Rename "Doc comments" to "Doc Comments", for consistency
  - Clarify that placing a doc comment in an unexpected place is a
    compiler error and add two incorrect examples
  - Document the current Autodoc behavior, when normal comments are
    interleaved with doc comments
  - Rewrite the documentation for top-level doc comments

* langref: improve the Zig Test section

Document that the test name can also be an identifier, in addition to a
string literal.

Update the test output for the first test.

Closes #14085