Fix static field documentation to clarify storage behavior for generic types

[Copilot]

The documentation incorrectly stated that there is "only one copy" of each static field, which is misleading for generic types. For generic types, each closed generic type (e.g., MyClass<int> vs MyClass<string>) has its own separate storage location for static fields.

Changes Made

Updated three documentation files to add clarifying language about static field storage in generic types:

• docs/csharp/language-reference/keywords/static.md - Added clarification after the statement about "only one copy of each static field"
• docs/csharp/programming-guide/classes-and-structs/static-classes-and-static-class-members.md - Added clarification in the "Static Members" section
• docs/csharp/misc/cs1914.md - Added clarification in the compiler error documentation

Example

class MyClass<T>
{
    static int count;
}

// These are separate storage locations:
MyClass<int>.count = 1;
MyClass<string>.count = 2;
// MyClass<int>.count is still 1, MyClass<string>.count is 2

The changes are minimal and surgical, preserving the existing content while adding the necessary clarification. The terminology continues to use "static field" as recommended by @BillWagner, avoiding the ambiguous "class field" term.

Fixes #22805.

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[jkotas]

Who about static fields marked with [ThreadStatic]? They have multiple copies too (one per thread).

[Copilot]

You're absolutely right! I've updated the documentation to also mention ThreadStatic fields. The text now clarifies that static fields marked with ThreadStaticAttribute have one copy per thread, in addition to the existing clarification about generic types. See commit 578c658.