minor #12440 Documented the String component (javiereguiluz)

This PR was squashed before being merged into the master branch (closes #12440).

Discussion
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Documented the String component

Fixes #12376 and #12404. This is an early WIP to test if this is correct.

@nicolas-grekas the full API is very long because there are a ton of methods ... so I thought about grouping them by "topic". Would you agree with that?

Thanks!

Commits
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9d08de4 Documented the String component

Author: javiereguiluz