Add information that `$method` is supported in AdGuard for iOS

[ameshkov]

There's an in-progress task of adding $method modifier support to AdGuard for iOS and we need to update the knowledge base accordingly.

Here's what needs to be done:

• Update the part about $method and add a note that it is supported by AdGuard for iOS starting with version v4.5.15 (and AdGuard Mini for macOS v2.1+): https://adguard.com/kb/general/ad-filtering/create-own-filters/#method-modifier
• Add a note about its limitation: $method in AdGuard for iOS and AdGuard Mini does not support negating, i.e. $method=~get is not supported.
• Update the compatibility table (supported with limitations: https://adguard.com/kb/general/ad-filtering/create-own-filters/#basic-rules-basic-modifiers