Document built-in HTML encoding design

HTML encoding principles have changed over time, but the changes are not completely reflected in the user guide.

Since 1.1 fieldValue and other tags should be sensitive to the default codec setting (GRAILS-3893), to avoid double encoding. Currently (Grails 1.3.6) standard generated views are HTML-encoded regardless of default codec setting "none" or "html", perhaps surprising to new users.

My theory (perhaps flaky) is that "${}" is not HTML-encoded if the default codec is "none". However, in such case HTML encoding in enclosed tags like fieldValue and textField kicks in. The roles are reversed if the default codec is "html", but the result is the same, the text is HTML encoded in both cases.

The behaviour and the design principles behind it should be documented in the user guide.