Improve exe `scope` related docs

(cherry picked from commit 261e5eb4)

Cabal/doc/developing-packages.rst

@@ -1246,6 +1246,12 @@ build information fields (see the section on `build information`_).
     must be relative to one of the directories listed in
     :pkg-field:`hs-source-dirs`.
 
+.. pkg-field:: scope: token
+
+    Whether the executable is ``public`` (default) or ``private``, i.e. meant to
+    be run by other programs rather than the user. Private executables are
+    installed into `$libexecdir/$libexecsubdir`.
+
 Running executables
 """""""""""""""""""
 

Cabal/doc/installing-packages.rst

@@ -658,7 +658,11 @@ path options:
 
 .. option:: --libexecsubdir=dir
 
-    A subdirectory of *libexecdir* in which private executables are installed.
+    A subdirectory of *libexecdir* in which private executables are
+    installed. For example, in the simple build system on Unix, the default
+    *libexecdir* is ``/usr/local/libexec``, and *libsubdir* is
+    ``x86_64-linux-ghc-8.0.2/mypkg-0.1.0``, so private executables would be
+    installed in ``/usr/local/libexec/x86_64-linux-ghc-8.0.2/mypkg-0.1.0/``
 
     *dir* may contain the following path variables: ``$pkgid``,
     ``$pkg``, ``$version``, ``$compiler``, ``$os``, ``$arch``, ``$abi``,