Doc: Clarify that text anchors carry no styling

The QTextCharFormat::setAnchor() documentation says that marking text
as an anchor formats it "in the appropriate way". A later paragraph
says the opposite: rendering is independent of the anchor. Nothing in
the layout or painting code reads the anchor property, so readers who
follow the first statement expect underlined, colored link text and
are surprised when nothing changes. The setAnchorHref() documentation
is also misleading. It claims the link target is shown as the display
text, and that setAnchorNames() changes it, but the target never
appears in the document and anchor names mark link destinations.

The documentation now states that an anchor doesn't change the text's
appearance, explains how to style link text, and notes that text
inserted as an HTML link gets link styling automatically. The
setAnchorHref() description now says the link applies to the
formatted text and that the target itself isn't shown.

Fixes: QTBUG-50303
Pick-to: 6.12
Change-Id: Ic2e0d5c68a0a61969b823ab01a4f986f7427c9eb
Reviewed-by: Topi Reinio <topi.reinio@qt.io>