docs(guide): update compiler guide with minor grammatical fixes

Minor changes to grammar. Changed sentence "But the declarative language
is also limited, since it does not allow you to teach the browser new syntax."
to now read "However, the declarative language is also limited, as it does not
allow you to teach the browser new syntax."
However is a less informal start to a sentence, and replacing "since"
correctly references extent/degree rather than comparison of time.