Improve the methods documentation comments in Code and in API reference

Improve the API reference for SDK 3.x

for example, in this getAsync method, there are no possible thrown exceptions that can be easily found without digging or experimenting in the code.

for example

GetAsync method could return DocumentNotFoundException, but also other exceptions, and there is no way of discovering it.

https://docs.couchbase.com/sdk-api/couchbase-net-client/api/Couchbase.KeyValue.ICouchbaseCollection.html#Couchbase_KeyValue_ICouchbaseCollection_GetAsync_System_String_Couchbase_KeyValue_GetOptions_

https://github.com/couchbase/couchbase-net-client/blob/381c0b7c57733565e714cccdd9c3167bba512ece/src/Couchbase/KeyValue/CollectionExtensions.cs#L17

Example:

https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/xmldoc/recommended-tags#exception