Doc: rm -none from constraint section, make installed prominent

- `PACKAGE -none` as already been removed in 3.4 as valid constraint.
(I guess this is now flag syntax.)

- Important constraint `installed` was just buried in an example, I made
it stand out more in a bullet point.
Same for `source`, even though this is an exotic constraint.

- Highlight constraint `installed` in connection with `allow-newer`.

- Group `test` and `bench` constraint with the `+/-FLAG` constraints.

Fixup: Apply concrete suggestions from code review

Fixes suggested by @ulysses4ever, thanks@

Co-authored-by: Artem Pelenitsyn <a.pelenitsyn@gmail.com>

doc/setup-commands.rst

@@ -880,37 +880,29 @@ Miscellaneous options
 .. option:: --constraint=constraint
 
     Restrict solutions involving a package to given version
-    bounds, flag settings, and other properties. For example, to
-    consider only install plans that use version 2.1 of ``bar``
-    or do not use ``bar`` at all, write:
+    bounds, flag settings, and other properties.
+
+    The following considers only install plans where ``bar``,
+    if used, is restricted to version 2.1:
 
     ::
 
         $ cabal install --constraint="bar == 2.1"
 
-    Version bounds have the same syntax as :pkg-field:`build-depends`.
-    As a special case, the following prevents ``bar`` from being
-    used at all:
+    The following prevents ``bar`` from being used at all:
 
     ::
 
-        # Note: this is just syntax sugar for '> 1 && < 1', and is
-        # supported by build-depends.
-        $ cabal install --constraint="bar -none"
-
-    You can also specify flag assignments:
+        $ cabal install --constraint="bar <0"
 
-    ::
-
-        # Require bar to be installed with the foo flag turned on and
-        # the baz flag turned off.
-        $ cabal install --constraint="bar +foo -baz"
+    Version bounds have the same syntax as :pkg-field:`build-depends`.
+    Yet extra pseudo version bounds are available here in addition:
 
-    To specify multiple constraints, you may pass the
-    ``constraint`` option multiple times.
+      - ``installed`` to fix a package to the already installed version.
+        Often useful for GHC-supplied packages in combination with :cfg-field:`allow-newer`,
+        e.g., ``--allow-newer='*:base' --constraint='base installed'``.
 
-    There are also some more specialized constraints, which most people
-    don't generally need:
+      - ``source`` to fix a package to the local source copy.
 
     ::
 
@@ -924,18 +916,30 @@ Miscellaneous options
         # specify this.)
         $ cabal install --constraint="bar source"
 
+    Further, we can specify flag assignments with ``+FLAG`` and ``-FLAG``
+    or enable test (``test``) and benchmark (``bench``) suites:
+
+    ::
+
+        # Require bar to be installed with the foo flag turned on and
+        # the baz flag turned off.
+        $ cabal install --constraint="bar +foo -baz"
+
         # Require that bar have test suites and benchmarks enabled.
         $ cabal install --constraint="bar test" --constraint="bar bench"
 
+    To specify multiple constraints, you may pass the
+    ``constraint`` option multiple times.
+
     By default, constraints only apply to build dependencies
     (:pkg-field:`build-depends`), build dependencies of build
     dependencies, and so on. Constraints normally do not apply to
     dependencies of the ``Setup.hs`` script of any package
     (:pkg-field:`custom-setup:setup-depends`) nor do they apply to build tools
     (:pkg-field:`build-tool-depends`) or the dependencies of build
     tools. To explicitly apply a constraint to a setup or build
-    tool dependency, you can add a qualifier to the constraint as
-    follows:
+    tool dependency, you can add a qualifier ``setup`` or ``any``
+    to the constraint as follows:
 
     ::