Clarification on the interaction between BaseAddress and RequestUri needed

There needs to be a clarification on how the BaseAddress and HttpRequestMessage's RequestUri are merged. Specifically, BaseAddress must end with a slash ('/') and RequestUri can't:

BaseAddress(BA) = https://server.domain.com/api
RequestUri(RU) = controller/action

Results in "https://server.domain.com/controller/action"

also:

BA = https://server.domain.com/api/
RU = /controller/action

Results in "https://server.domain.com/controller/action" too

and finally:

BA = https://server.domain.com/api/
RU = controller/action

Results in "https://server.domain.com/api/controller/action" which is the expected request

Documentation should explain this behavior and not assume people being familiar with RFC 3986 (https://tools.ietf.org/html/rfc3986)

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* ID: 9294a1a6-2e97-10b4-ddf9-79893a343ee5
* Version Independent ID: de1d10bd-b17f-bad7-9409-87323035ff10
* Content: [HttpClient.BaseAddress Property (System.Net.Http)](https://docs.microsoft.com/en-us/dotnet/api/system.net.http.httpclient.baseaddress?redirectedfrom=MSDN&view=netframework-4.7.2#System_Net_Http_HttpClient_BaseAddress)
* Content Source: [xml/System.Net.Http/HttpClient.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Net.Http/HttpClient.xml)
* Product: **dotnet-api**
* GitHub Login: @karelz
* Microsoft Alias: **ncldev**