Clarify MidpointRounding docs: round-to-nearest vs directed rounding

[Copilot]

The MidpointRounding name implies every mode only affects midpoints, but three of its five modes (ToNegativeInfinity, ToPositiveInfinity, ToZero) are directed-rounding modes that round every value in a fixed direction. Resolves dotnet/runtime#38160, which covers the current docs bury this distinction and the individual field summaries don't call it out.

Changes to xml/System/MidpointRounding.xml:

• Type remarks: Added an upfront note that despite the enum name not every mode is midpoint-only, followed by a two-bullet summary contrasting the round-to-nearest and directed strategies before the existing per-strategy sections.
• Directed rounding section: Reworded to explicitly state these modes apply to every input, with a concrete example (ToPositiveInfinity rounds 2.1, 2.5, and 2.8 all up to 3).
• AwayFromZero / ToEven summaries: Noted that non-midpoint values are rounded to nearest regardless of the choice; added "banker's rounding" alias for ToEven.
• ToNegativeInfinity / ToPositiveInfinity / ToZero summaries: Made explicit that every value (not just midpoints) is rounded in the given direction, and referenced the familiar equivalents (floor, ceiling, truncation).

[Copilot]

Pull request overview

This PR clarifies the documentation for System.MidpointRounding by explicitly distinguishing round-to-nearest modes (only differ at midpoints) from directed rounding modes (apply to all values), addressing confusion stemming from the enum’s name.

Changes:

• Adds an upfront explanation in the type remarks contrasting round-to-nearest vs directed rounding.
• Strengthens the directed-rounding section with explicit “applies to every value” wording and a concrete example.
• Updates individual enum-field summaries to call out midpoint vs non-midpoint behavior and adds common aliases (e.g., “banker’s rounding”).

Show a summary per file

File	Description
xml/System/MidpointRounding.xml	Refines the enum remarks and field summaries to better explain which modes affect only midpoints vs all values.

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 6