Clarify ref docs around NextAsync.

[JonathanFingold]

To address bot-docs issue 1193.

[coveralls]

Pull Request Test Coverage Report for Build 63541

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage remained the same at 71.155%

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Files with Coverage Reduction	New Missed Lines	%
/libraries/Microsoft.Bot.Builder.Dialogs/WaterfallStepContext.cs	2	86.96%

Totals
Change from base Build 63235:	0.0%
Covered Lines:	4344
Relevant Lines:	6105

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💛 - Coveralls

[fuselabs]

✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.dll
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.AI.Luis.dll
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.AI.QnA.dll
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.ApplicationInsights.dll
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Azure.dll
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Dialogs.dll
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.TemplateManager.dll
✔️ No Binary Compatibility issues for Microsoft.Bot.Configuration.dll
✔️ No Binary Compatibility issues for Microsoft.Bot.Connector.dll
✔️ No Binary Compatibility issues for Microsoft.Bot.Schema.dll