Wrong parameter type for term_id in get_term_children

[wido]

The parameter $term_id must be an int value for get_term_children() but the doc say string.

Internally the variable is converted to an int using the function intval that is correct since we cannot take advantage of php7 scalar type declaration but the correct value to pass to the function must be an integer not a string.

[Soean]

Changes $term_id @param docs from string to int

[SergeyBiryukov]

In 41235:

Docs: Correct @param type for $term_id in get_term_children().

Props Soean, wido.
Fixes #41585.