Add `augment` option to `doc_header`

[ablaom]

From the new doc string for doc_header:

---

Variation to augment existing document string

For models that have a native API with separate documentation, one may want to call doc_header(FooRegressor, augment=true) instead. In that case, the output will look like this:

From MLJ, the FooRegressor type can be imported using

FooRegressor = @load FooRegressor pkg=FooRegressorPkg

Construct an instance with default hyper-parameters using the syntax model = FooRegressor(). Provide keyword arguments to override hyper-parameter defaults, as in FooRegressor(a=...).

To prevent an existing document string being replaced instead of augmented, the doc_header declaration must appear in a different module from the original.

[codecov-commenter]

Codecov Report

Merging #168 (97ffecf) into dev (100c477) will increase coverage by 0.03%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #168      +/-   ##
==========================================
+ Coverage   88.70%   88.73%   +0.03%     
==========================================
  Files           8        8              
  Lines         354      355       +1     
==========================================
+ Hits          314      315       +1     
  Misses         40       40              

Impacted Files	Coverage Δ
src/metadata_utils.jl	98.38% <100.00%> (+0.02%)	:arrow_up:

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