Document Base64Url character encoding details in API reference

[Copilot]

This PR enhances the documentation for the System.Buffers.Text.Base64Url class to clearly specify which characters are used in Base64Url encoding, addressing a documentation gap identified by users.

Changes Made

Class-level documentation:

• Updated the <remarks> section to explain that Base64Url uses the same alphabet as standard Base64 encoding, except that the characters + and / are replaced with - and _ respectively to make the output URL-safe.

Method-level documentation:

• Enhanced the <remarks> section for all encoding methods (EncodeToString, EncodeToChars, EncodeToUtf8, and their overloads) to mention the specific character replacements.
• Applied changes consistently across all 10 encoding method overloads to maintain API documentation consistency.

Before

The documentation only mentioned "URL-safe ASCII encoded text" and "Base64Url characters" without specifying what makes it URL-safe or what the actual character set differences are.

After

The documentation now clearly explains:

• What characters replace the standard Base64 + and / characters (namely - and _)
• Why these replacements make the encoding URL-safe
• This information is consistently documented across all relevant methods

This change helps developers understand the practical differences between standard Base64 and Base64Url encoding without having to reference external specifications.

Fixes #11469.

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