docs: mark previously unmarked test-covered manual examples

[tueda]

This patch adds missing test-suite annotations to 5 manual examples covered by check/examples.frm and documents the related negative-test variant (Sta_Transform_11b). They were found while using AI to check for other issues.

The test and annotation counts can be found with:

$ ./check/check.rb examples.frm -l | tail -1  # 127 annotated tests + 1 negative test
128 tests

$ rg 'THIS EXAMPLE IS PART OF THE TESTSUITE' doc | wc -l  # annotations including duplicated ones
130

$ rg --no-filename 'WELL!' doc | sort | uniq | wc -l  # unique annotations
127

[coveralls]

coverage: 65.048% (-0.03%) from 65.081% — tueda:pr/docs/mark-tested-examples into form-dev:master