Document UseCurrentRuntime

Initial Feature was added in 2020 but not documented here: dotnet/sdk#10449
We've added implicit RIDs here: dotnet/sdk#26143
And we've added interaction to toggle implicit RIDs off with UCR here: dotnet/sdk#28628 
And we added the shorthand for ucr here: dotnet/sdk#27971 

Needs to be duplicated for `dotnet publish` as well.

docs/core/tools/dotnet-build.md

@@ -19,7 +19,7 @@ dotnet build [<PROJECT>|<SOLUTION>] [-a|--arch <ARCHITECTURE>]
     [--force] [--interactive] [--no-dependencies] [--no-incremental]
     [--no-restore] [--nologo] [--no-self-contained] [--os <OS>]
     [-o|--output <OUTPUT_DIRECTORY>] [-r|--runtime <RUNTIME_IDENTIFIER>]
-    [--self-contained [true|false]] [--source <SOURCE>]
+    [--self-contained [true|false]] [--source <SOURCE>] [--use-current-runtime, --ucr [true|false]]
     [-v|--verbosity <LEVEL>] [--version-suffix <VERSION_SUFFIX>]
 
 dotnet build -h|--help
@@ -134,6 +134,10 @@ The project or solution file to build. If a project or solution file isn't speci
 
 [!INCLUDE [verbosity](../../../includes/cli-verbosity-minimal.md)]
 
+- **`--use-current-runtime, --ucr [true|false]`**
+
+  Sets the `RuntimeIdentifier` to a platform portable `RuntimeIdentifier` based on the one of your machine. This happens implicitly with properties that require a `RuntimeIdentifier`, such as `SelfContained`, `PublishAot`, `PublishSelfContained`, `PublishSingleFile`, and `PublishReadyToRun`. If the property is set to false, that implicit resolution will no longer occur.
+
 - **`--version-suffix <VERSION_SUFFIX>`**
 
   Sets the value of the `$(VersionSuffix)` property to use when building the project. This only works if the `$(Version)` property isn't set. Then, `$(Version)` is set to the `$(VersionPrefix)` combined with the `$(VersionSuffix)`, separated by a dash.