doc: clarify that the "-j N" goes after the "--build build" part

Also, capitalized the comments in build-unix.md for uniformity
with the docs on other targets.

doc/build-osx.md

@@ -182,8 +182,8 @@ After configuration, you are ready to compile.
 Run the following in your terminal to compile Bitcoin Core:
 
 ``` bash
-cmake --build build     # Use "-j N" here for N parallel jobs.
-ctest --test-dir build  # Use "-j N" for N parallel tests. Some tests are disabled if Python 3 is not available.
+cmake --build build     # Append "-j N" here for N parallel jobs.
+ctest --test-dir build  # Append "-j N" for N parallel tests. Some tests are disabled if Python 3 is not available.
 ```
 
 ### 3. Deploy (optional)