Correcting documentation for span and memory slice

[Anipik]

Fixes https://github.com/dotnet/corefx/issues/35063

We only throw exceptions when the end index of sliced\AsSpan\Ctor query is greater than length. when the index = length, we just return an empty span

[ahsonkhan]

LGTM. However, we should make this change in the api-docs repo itself for each instance this is incorrect. For example:
https://docs.microsoft.com/en-us/dotnet/api/system.span-1.slice?view=netcore-3.0#System_Span_1_Slice_System_Int32_

index is less than zero or greater than or equal to Length.

1. It should be start, not index
2. It should be greater than Length, (not greater than or equal)

See issue dotnet/dotnet-api-docs#1771

cc @mairaw