Clean up property behavior and doc comments for AzureAppServicesDiagnosticsSettings

[Eilon]

Some random stuff I noticed:

1. The property setters should throw ArgOutOfRange w/ appropriate message for invalid values
2. BackgroundQueueSize has no proper doc comment
3. The non-doc comment comment for BackgroundQueueSize has a typo: Gets of sets ... --> Gets or sets ...
4. Would probably be nice for the doc comments to indicate the actual default values, e.g. The default value for this property is <c>FOO</c>. or whatever the right doc comment syntax is.

@muratg we should do this for 1.1.0.

[Eilon]

(Or whatever version this thing is.)

[muratg]

• Fix package description for Microsoft.Extensions.Logging.AzureAppServices #507 & Fix exception messages for Azure App Services logger to say "must" instead of "should" #508