Add missing SaveAsync method

[JimBobSquarePants]

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• I have provided test coverage for my change (where applicable)

Description

Adds the missing Image.SaveAsync(Stream, IImageformat, CancellationToken) extension.

See https://stackoverflow.com/q/63639644/427899

I also cleaned up some bad copy/paste in the extension method documentation.

[codecov]

Codecov Report

Merging #1334 into master will increase coverage by 0.02%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master    #1334      +/-   ##
==========================================
+ Coverage   82.75%   82.77%   +0.02%     
==========================================
  Files         689      689              
  Lines       30938    30956      +18     
  Branches     3508     3511       +3     
==========================================
+ Hits        25602    25625      +23     
+ Misses       4617     4610       -7     
- Partials      719      721       +2     

Flag	Coverage Δ
#unittests	82.77% <60.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/ImageSharp/Advanced/AdvancedImageExtensions.cs	60.60% <ø> (ø)
src/ImageSharp/ImageExtensions.cs	64.44% <46.66%> (-10.56%)	⬇️
...ssing/Processors/Convolution/BokehBlurProcessor.cs	100.00% <100.00%> (ø)
src/ImageSharp/Formats/Gif/GifDecoder.cs	82.35% <0.00%> (+23.52%)	⬆️
src/ImageSharp/Formats/Png/PngDecoder.cs	83.33% <0.00%> (+33.33%)	⬆️
src/ImageSharp/Formats/Tga/TgaDecoder.cs	84.61% <0.00%> (+38.46%)	⬆️

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