Html: Make linkButton() documentation less confusing

Html: Make linkButton() documentation less confusing

The documentation for the first parameter said it was both "raw HTML"
and "not escaped", but those phrases have opposite meanings. Clarify that
this parameter takes plain text that will be escaped (so not raw HTML),
and rename the parameter from $contents (typically used to mean HTML) to
$text.

Change-Id: I44d6de08a68c493935b2ceff58fac6c51bc45d12