docs(wiki): document generic classes in @generic section

[xiangnanscu]

Description

The @generic section still says generics are limited to functions and index-signature classes (Array<T>: { [integer]: T }), and does not mention the generic-class features that landed in v3.17.0:

• generic class inheritance with type arguments (#1929)
• method return type resolution on generic classes (#1863)
• generic type parameters in @overload (#723)
• inline fun<T> generic function types in @field/@type (#1170)

This PR:

• adds one sentence to the section intro listing the v3.17.0 additions
• adds a Generic Class with Fields and Methods example, covering ---@class Box<T> with ---@field value T / ---@field get fun(self: Box<T>): T, and inheritance from an instantiated generic class (---@class IntegerBox: Box<integer>), with a note describing when the type parameter is substituted (members declared as ---@field entries substitute both on direct instantiation and through inheritance; separately declared methods only substitute when the receiver's type is the instantiated generic class itself)
• adds an Inline Generic Function Type example (---@field identity fun<T>(value: T): T)

Verification

Every example was verified against lua-language-server 3.18.2 by running lua-language-server --check over probe files that assign each documented expression to an intentionally wrong type and asserting the expected assign-type-mismatch diagnostics appear (and that no diagnostics appear where substitution is documented to work).

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