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-freebsd.md

@@ -95,6 +95,6 @@ cmake -B build -DENABLE_WALLET=OFF
 ### 2. Compile
 
 ```bash
-cmake --build build     # Use "-j N" 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" 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.
 ```