Squashed commit of the following: Fix issue 42

Supersedes earlier pull request (which I will close now).

commit 30680aeabad159ab79d1af2e3f29ea67290fdcec
Author: Adrian Aichner <adrian.aichner@gmail.com>
Date:   Tue Nov 26 13:08:38 2013 +0100

    Clarify reason for use of innerText.
    Improve wording of a warning.

    Untangle from issue35 with "git rebase -i 67ecc84"

commit 69c19e6b1ec257ee30989b7cfd786b07f5d9bbfe
Author: Adrian Aichner <adrian.aichner@gmail.com>
Date:   Thu Nov 7 00:51:41 2013 +0100

    Improve text content restriction warning.

commit ade1c6db8316a15736576542e2a6a48d7b03b090
Author: Adrian Aichner <adrian.aichner@gmail.com>
Date:   Wed Nov 6 19:09:42 2013 +0100

    Warn about *[i18n-content] elements having child elements (not supported).

    Pushing HTML content markup to chrome.i18n message files would be a bad idea.

    Add warning to applyChromeI18nMessages.js when chrome.i18n is not available, but tolerate it (using chrome extension pages stand-alone for content  development).