[mdoc] Explicitly order //Member children.

[mdoc] Explicitly order //Member children.

The problem is the intermixing of updating "old" documentation and
wanting the result to pass `mdoc validate`.

For example, mdoc-2.6 will only generate a <MemberSignature/> for C#,
while mdoc-2.8+ will generate two, one for C# and one for ILAsm.
Unfortunately, mdoc will _append_ the new <MemberSignature/> as the
_last_ child within <Member/>; result: 'mdoc validate' gives errors.

Oops.

Thus, we explicitly reorder //Member children so that the ordering
matches the order in monodoc-ecma.xsd.