Improve definitions and examples in the Grammar and mechanics guide regarding periods

[marijnschraagen]

Is your feature request related to a problem? Please describe.
The Grammar and mechanics guide contains a section on when to use periods: https://stackoverflow.design/content/guidelines/grammar-and-mechanics/#periods. It states:

Don’t use periods in interface copy unless it’s a full sentence or description.

However, the section does not contain a definition on what a "full sentence" is (or "complete sentence" as it is called in the examples in the section). This leads to, or at least does not address, meta questions on the usage of periods on SE.

Also, the examples provided in different parts of the guide seem inconsistent regarding period use. For example from Verbs and nouns, no period:

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Contrast with Hyphens (with period):

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Describe the solution you'd like
Add a definition of what a "full sentence" or "complete sentence" is (or what it is not). A useful difference seems to be an imperative phrase (do this) vs an indicative sentence, i.e., a "regular" sentence with at minimum a subject and a verb (Your account has been suspended.). Also choose between the two terms, either "full sentence" or "complete sentence" but not both.

The guide should also mention the difference between single phrases and combinations of two or more phrases, in which each phrase should end with a period, even if it is an imperative phrase.

Explicit examples for each case should be added in the Periods section, for the following cases:

• imperative phrase
• combination of two or more imperative phrases
• complete sentence

Current incorrect examples should be changed, for example:

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Your job listing was published on January 15

Additional context
This feature request was prompted by https://meta.stackexchange.com/questions/380566/make-full-stop-usage-consistent-in-badge-descriptions/380592.