doc: add FTS5 requirement for sqlite backend. Document the requirement for FTS5 if you are using the sqlite backend. Tonu Mikk reported issues with using a SQLite that was missing FTS5 support (RedHat 7 sqlite with python 3.6). Even though FTS5 has been in the sqlite amalgamation since 2015-10-14 (v3.9.0) RedHat chose to not build their package with that. This leads to a traceback when initializing the database.

try:
    from html import escape as html_escape_  # python 3

    def html_escape(str, quote=False):
        # html_escape under python 3 sets quote to true by default
        # make it python 2 compatible
        return html_escape_(str, quote=quote)
except ImportError:
    # python 2 fallback
    from cgi import escape as html_escape  # noqa: F401