Made a number of updates and additions to docs/INSTALL.html

• Added a section on anti-spam measures (a new section 7, and renumbered the existing Disclaimer section as "8") in response to PHPBB3-10861.

• Changed references from phpBB3 to phpBB (49 instances). It is unlikely that people now installing phpBB would not be clear that it is version 3 and as far as a I can tell phpBB3 was only ever intended to differentiate phpBB3 from phpBB2.

• Changed www.mydomain,com or www.mydomain.tld to www.example.com (5 instances) since www.mydomain.com is the website of an actual domain registrar.

• Added a paragraph to the Updating section on the need to also update 3rd party styles

• Reworded a few sentences to, hopefully, make the meaning clearer.

• Changed the formatting of file names and paths for more consistency, before there was a mix of <bold>, <em>, <samp> and <code> tags for both files names and paths.

• Updated references to 3.0.9 to 3.0.10 and 3.0.10 to 3.0.11