feat(Forms): Add docs for input description text

What does this MR do?

This MR adds documentation for the (optional) description field added to text inputs within the form component. See Text inputs: Add description field for more context. The problem it solves is that the help text didn't work for SAST Configuration UI because:

• Help text needs to be contained to the width of the input container, and we need 1-2 sentences to explain the variable
• We're using the help text for another purpose; if the user changes the variable, we need to tell them that they will be excluded from template updates, and then we apply a link to revert if they so choose

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Links

#642 (closed) #723 (closed)