Added deprecation note about 'virtual' form option. Issue #12603.
minor #12629 Added deprecation note about 'virtual' form option. Issue #12603. (Lenard Palko)

This PR was squashed before being merged into the 2.7 branch (closes #12629).

Discussion
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Added deprecation note about 'virtual' form option. Issue #12603.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #12603
| License       | MIT
| Doc PR        |

Uncommented the trigger_error method calls for 'virtual' form option.

Commits
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1308310 Added deprecation note about 'virtual' form option. Issue #12603.