Improve documentation

Problem/Motivation

As stated in the quotes above, we need to document the expected results of this filter.

Should we document that if there is a mix of text in paragraph tags and not in paragraphs tags, the content that is not in paragraph tags will be lost? (That's how it's going to work here, correct?)

@mlncn in #3410145-9: Paragraph tags are not stripped if the p tag has attributes like dir=ltr or title →

I'd say it needs documenting, because it's a change that makes this module impossible for me to use. I was hoping to use it as a workaround for a TMGMT issue where my text sometimes is saved with <p> wraparounds and sometimes not, and this means it won't work in that situation. Obviously that's not the fault of this module, and the right fix in my situation is to get TMGMT to quit taking formatted fields with textfield widgets and presenting them as textareas with CKEditor, but it would have saved me some time to know that a mix of <p> and no <p> is unsupported.

@ksenzee in #3410145-10: Paragraph tags are not stripped if the p tag has attributes like dir=ltr or title →