PHPDoc parser: Removed p tags and added whitespace instead.

[zinigor]

Fixes a problem where certain hooks would get words in the description squashed together because of missing spaces. The strip_all_tags function that outputs them would remove the </p><p> group. This change adds a whitespace instead and removes p tags entirely because they get stripped anyway.

Proposed changes:

• Replaces p tags with whitespaces.

Other information:

• Have you written new tests for your changes, if applicable?
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Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

• Go to '..'

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[tbradsha]

If you wanted to be really picky, you could also trim() the description, but not important.