What type `bound` parameter of `typing.TypeVar` accepts should be explained more clearly

Documentation

*Notes:

• Python 3.14.3

For T and A, what type is accepted is clearly explained in the example of typing.TypeVar while for S isn't explained clearly even though str itself is also accepted as shown below:

T = TypeVar('T')  # Can be anything
S = TypeVar('S', bound=str)  # Can be any subtype of str <- Here
A = TypeVar('A', str, bytes)  # Must be exactly str or bytes

So, S should be explained more clearly as shown below:

S = TypeVar('S', bound=str)  # Can be str or any subtype of str

Linked PRs

• gh-145525

從 typing.TypeVar 文件以及 issue 中連結的 T、S 和 A 範例開始。釐清 bound=str 範例，明確說明 str 本身及其子型別都可接受，然後檢查轉譯後的文件，確保措辭一致。