[doc] Missing doc for option `strut`

In package caption, version 2020-01-01,

• Option strut is implemented, and by default strut=true. Therefore extra vertical skip (compared to the latex2e kernel) is added above and below the caption text (inside definition of \caption@@@make).
• Option strut is listed in the doc, Sec. A.1 Options.
• The Sec. A.1 also shows strut is documented at Sec. 2.1, but there are not any introduction to this option there.
• The word strut appears in an example in Sec. 6.2, and there are no other occurrences of this word throughout the documentation.

Hence, there may need more introduction to option strut in the user documentation.