Update PACKAGE_MANUAL option

Explain that a prebuilt copy of the manual must be present under 'manual/'
in the build directory.

Signed-off-by: Avery King <avery98@pm.me>

BUILDING.md

@@ -315,8 +315,9 @@ too if you are using vcpkg. If not, set this to an empty string. Next, run
   * **PERFORM_NOTARIZATION** (ON|OFF): [macOS Only] Performs notarizaiton during
     the install step. This only works on macOS and if PERFORM_NOTARIZATION has
     been enabled.
-  * **PACKAGE_MANUAL** (ON|OFF): Packages the (Audacity) manual alongside DMG and
-    InnoSetup targets.
+  * **PACKAGE_MANUAL** (ON|OFF): Packages the manual alongside DMG and
+    InnoSetup targets (requires a prebuilt manual to be present under 'manual/'
+    in the build directory.
 
 The following feature options are enabled by default if the required libraries
 are found. You may explicitly disable them if you prefer or your distribution