Add XML documentation to generic FragmentManager helper methods

[Copilot]

FindFragmentById<T>, FindFragmentByTag<T>, and GetFragment<T> in FragmentManager.cs are public shipped API with no /// XML docs, producing blank IntelliSense summaries and no learn.microsoft.com content.

Changes

• src/Mono.Android/Android.App/FragmentManager.cs — added complete <summary>, <typeparam>, <param>, and <returns> doc comments to all three generic helper methods; no code changes.

/// <summary>
/// Finds a <see cref="Fragment"/> that was identified by the given id, either when
/// inflated from XML or as the container id when added in a transaction, and returns
/// it cast to <typeparamref name="T"/>.
/// </summary>
/// <typeparam name="T">The <see cref="Fragment"/> type to cast the result to.</typeparam>
/// <param name="id">The resource id or container id used to identify the fragment.</param>
/// <returns>
/// The matching fragment cast to <typeparamref name="T"/>, or <see langword="null"/>
/// if no matching fragment exists.
/// </returns>
public T? FindFragmentById<
        [DynamicallyAccessedMembers (Constructors)]
        T
> (int id)
    where T : Fragment