Fix misleading docs for ActionLink and PageLink

[GrantTotinov]

Fix misleading XML docs for ActionLink and PageLink

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Summary of the changes (Less than 80 chars)
Fix misleading XML docs for ActionLink and PageLink

Description

ActionLink and PageLink always generate an absolute URL. When protocol
or host are null, the values are taken from the current HTTP request.
The previous documentation incorrectly implied that an absolute URL is
only generated when both parameters are non-null.

Updated the summary comments for both methods to reflect the actual behavior.

Fixes #42949

[Copilot]

Pull request overview

Updates MVC UrlHelperExtensions XML documentation to clarify that ActionLink and PageLink generate absolute URLs by defaulting protocol and host from the current HTTP request when those parameters are null.

Changes:

• Update ActionLink summary to state it generates an absolute URL and defaults protocol/host from the current request when null.
• Update PageLink summary to state it generates an absolute URL and defaults protocol/host from the current request when null.