Include docs for with re-exported functions with -femit-docs.

[nmichaels]

An example: math.zig includes the following code:

pub const isSignalNan = @import("math/isnan.zig").isSignalNan;

The generated docs for the math namespace show no documentation for this function. However, in math/isnan.zig, there's this:

/// Returns whether x is a signalling nan.
pub fn isSignalNan(x: var) bool {

Which is exactly where the documentation for the function ought to be. I'm proposing that the generated documentation should, when displaying a symbol that's just re-exported from a different Zig module in this rather common idiom, include that symbol's documentation.

[nmichaels]

fixed by #19208