fix: Add overloads for doc() and doc(id:string)

[schmidt-sebastian]

This addresses the same issue as #658, but instead of relaxing our CollectionReference.doc() API, it fixes the types to reflect the invariant that we enforce via our argument validation. As in older releases, we still prevent users from passing "undefined", but with this PR this should generate a compile time error.

This is based on the following little experiment:

interface Foo {
    bar(): void;
    bar(baz: string): void;
}
class Foobar implements Foo {
    bar(baz?: string) {}
}
function createFoo(): Foo {
    return new Foobar();
}
const f = createFoo();
f.bar();
f.bar("foobar");
f.bar(undefined); // Compile error

Note that I am marking this as a "fix" rather than as a "breaking change" as any user that tried to use "undefined" before would have been broken already (and gotten a validation error). This PR will therefore go out with the next release, which is a minor version bump.

[schmidt-sebastian]

This weird arrangement here is so that our JSDoc generation still works. The TypeScript compiler still validates that we do not pass "undefined" (hence the test change).

[codecov]

Codecov Report

Merging #662 into master will decrease coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #662      +/-   ##
==========================================
- Coverage   61.55%   61.46%   -0.09%     
==========================================
  Files          21       21              
  Lines        3418     3418              
  Branches      459      459              
==========================================
- Hits         2104     2101       -3     
- Misses       1252     1254       +2     
- Partials       62       63       +1

Impacted Files	Coverage Δ
dev/src/reference.ts	99.01% <ø> (ø)	⬆️
dev/src/watch.ts	96.52% <0%> (-1.16%)	⬇️

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[codecov]

Codecov Report

Merging #662 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #662   +/-   ##
=======================================
  Coverage   61.54%   61.54%           
=======================================
  Files          21       21           
  Lines        3425     3425           
  Branches      460      460           
=======================================
  Hits         2108     2108           
  Misses       1254     1254           
  Partials       63       63

Impacted Files	Coverage Δ
dev/src/reference.ts	99.01% <ø> (ø)	⬆️

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[mikelehen]

LGTM. Thanks!