--make docs blend better with the surrounding

Plus some more cleanup of the usage docs.

docs/usage.rst

@@ -98,16 +98,18 @@ commands are implemented.
 
 .. _PGXS: http://www.postgresql.org/docs/current/static/extend-pgxs.html
 
-PGXS_ relies on a presence of GNU Make — in most systems it's
-usually installed as ``gmake`` or ``make`` executable. If it's not the case you
-should provide path to GNU Make (or just simply a different executable name)
-using ``--make`` option.
-
 If there are many PostgreSQL installations on the system, the extension will
 be built and installed against the instance whose :program:`pg_config` is
 first found on the :envvar:`PATH`. A different instance can be specified using
 the option :samp:`--pg_config {PATH}`.
 
+The PGXS_ build system relies on a presence of `GNU Make`__: in many systems
+it is installed as :program:`gmake` or :program:`make` executable. The program
+will use the first of them on the path. You can specify an alternative program
+using ``--make`` option.
+
+.. __: http://www.gnu.org/software/make/
+
 If the extension is being installed into a system PostgreSQL installation, the
 install phase will likely require root privileges to be performed.  In this
 case either run the command under :program:`sudo` or specify the ``--sudo``
@@ -166,6 +168,8 @@ The database connection options are similar to the ones in load_, with the
 difference that the variable :envvar:`PGDATABASE` doesn't influence the
 database name.
 
+See the install_ command for details about the command arguments.
+
 .. warning::
     At the time of writing, :program:`pg_regress` on Debian and derivatives is
     affected by `bug #554166`__ which makes *HOST* selection impossible.
@@ -257,7 +261,7 @@ confirmation.
 
 If the distribution provides more than one extension, the extensions are
 loaded in the order in which they are specified in the ``provides`` section of
-the ``META.json`` file. It is also possilbe to load only a few of the
+the ``META.json`` file. It is also possible to load only a few of the
 extensions provided, specifying them after *SPEC*: the extensions will be
 loaded in the order specified.
 
@@ -305,7 +309,7 @@ itself, so the option will be ignored.
 
 If the distribution specifies more than one extension, they are unloaded in
 reverse order respect to the order in which they are specified in the
-``META.json`` file.  It is also possilbe to unload only a few of the
+``META.json`` file.  It is also possible to unload only a few of the
 extensions provided, specifying them after *SPEC*: the extensions will be
 unloaded in the order specified.