Clarify Enumerable.Prepend returns a new sequence

[oii-nasif]

Summary

• Updated Enumerable.Prepend<TSource> summary to clarify it returns a new sequence
• Changed from "Adds a value" (implies modification) to "Returns a new sequence" (accurate behavior)

Details

The Enumerable.Prepend<TSource> method had a confusing summary that stated "Adds a value to the beginning of the
sequence." This wording incorrectly implied that the method modifies the original sequence in place.

The Problem

The original summary was misleading because:

• "Adds a value" suggests in-place modification
• Developers might expect the source sequence to be changed
• This contradicts LINQ's core principle of deferred execution and immutability

While the Remarks section included a note clarifying "This method does not modify the elements of the collection.
Instead, it creates a copy of the collection with the new element," the summary itself was still confusing and
contradictory.

Changes Made

Updated the summary from:

Adds a value to the beginning of the sequence.

To:

Returns a new sequence that begins with the specified element followed by the elements of the source sequence.

Why This Matters

This clarification helps developers:

• Understand that Prepend follows LINQ's immutability pattern
• Avoid expecting the source collection to be modified
• Write correct code that properly handles the returned sequence
• Align with how other LINQ methods like Append, Select, Where, etc. are documented

The new wording is consistent with the Returns section which already correctly stated "A new sequence that begins
with element."

Fixes #11745

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-linq