Provider init callback needs to be documented

[tetromino]

If a provider has init specified, we want to document it.

In other words,

def _my_info_init(a, c = "hello"):
    """
    MyInfo constructor

    Args:
          a: Value of field a
          c: Value to frobnicate
    """
    return {"a": a, "b": frobnicate(c)}

MyInfo = provider(
    doc = """This is my provider""",
    fields = {
        "a": "Field a",
        "b": "Field b (frobnicated)",
    },
    init = _my_info_init,
)

should get documented as something like

---

MyInfo

This is my provider

MyInfo(a, c)

MyInfo constructor

CONSTRUCTOR PARAMETERS

Name	Description	Default Value
a	Value of field a	none
c	Value to frobnicate	"hello"

FIELDS

Name	Description
a	Field a
b	Field b (frobnicated)

---

This is a prerequisite for switching to stardoc / starlark_doc_extract for generating Bazel's own documentation.