Custom TimeSpan Format

It would be nice to add a small note to link to [this page](https://docs.microsoft.com/en-us/dotnet/standard/base-types/custom-timespan-format-strings) in the remarks section.
There is the complete list of symbols, yet not a word about how to escape them.

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#### Document Details

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* ID: 26b52684-e2ce-3140-893f-1230bc2eeb3d
* Version Independent ID: 082096cb-4250-a700-725d-c8f99eb96ff8
* Content: [TimeSpan.TryParse Method (System)](https://docs.microsoft.com/en-us/dotnet/api/system.timespan.tryparse?view=xamarinios-10.8)
* Content Source: [xml/System/TimeSpan.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System/TimeSpan.xml)
* Product: **dotnet-api**
* GitHub Login: @dotnet-bot
* Microsoft Alias: **dotnetcontent**