[github] Document what the macOS/Linux pull requests do in the PR template for easy access.

Author: gottesmm

.github/PULL_REQUEST_TEMPLATE.md

@@ -25,6 +25,24 @@ The swift-ci is triggered by writing a comment on this PR addressed to the GitHu
         OS X platform               | @swift-ci Please smoke test OS X platform
         Linux platform              | @swift-ci Please smoke test Linux platform
 
+A smoke test on macOS does the following:
+
+1. Builds the compiler incrementally.
+2. Builds the standard library only for macOS. Simulator standard libraries and
+   device standard libraries are not built.
+3. lldb is not built.
+4. The test and validation-test targets are run only for macOS. The optimized
+   version of these tests are not run.
+
+A smoke test on Linux does the following:
+
+1. Builds the compiler incrementally.
+2. Builds the standard library incrementally.
+3. lldb is build incrementally.
+4. The swift test and validation-test targets are run. The optimized version of these
+   tests are not run.
+5. lldb is tested.
+
 **Validation Testing**
 
         Platform     | Comment