HTMLForm: Improve method documentation for setCollapsibleOptions()

HTMLForm: Improve method documentation for setCollapsibleOptions()

Follows-up 36d33daa03a63.

Previously the method was described as "Set whether the form can be collapsed"
taking a single boolean.

Yet, the boolean did not influence whether the form can be collapsed, as one
might expect.

Rather, this method always enable collapsible mode. The thing that is set
based on the passed value is the default state of that (unconditionally)
enabled collapsible mode.

Change-Id: I2e73d5481c44ed43769553b2bc25543a702c19b8