docs/manual: dependency documentation updates

Update dependency documentation to detail the order-only relationship
associated with the DEPENDENCIES variable.  See the thread at [1] for
details.

[1] http://lists.busybox.net/pipermail/buildroot/2019-October/262685.html

Signed-off-by: Dan Walkes <danwalkes@trellis-logic.com>
[yann.morin.1998@free.fr: indentation & slight rephrasing]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

docs/manual/adding-packages-generic.txt

@@ -358,9 +358,11 @@ not and can not work as people would expect it should:
 * +LIBFOO_DEPENDENCIES+ lists the dependencies (in terms of package
   name) that are required for the current target package to
   compile. These dependencies are guaranteed to be compiled and
-  installed before the configuration of the current package starts. In
-  a similar way, +HOST_LIBFOO_DEPENDENCIES+ lists the dependencies for
-  the current host package.
+  installed before the configuration of the current package starts.
+  However, modifications to configuration of these dependencies will
+  not force a rebuild of the current package. In a similar way,
+  +HOST_LIBFOO_DEPENDENCIES+ lists the dependencies for the current
+  host package.
 
 * +LIBFOO_EXTRACT_DEPENDENCIES+ lists the dependencies (in terms of
   package name) that are required for the current target package to be

docs/manual/rebuilding-packages.txt

@@ -65,6 +65,16 @@ can help you understand how to work with Buildroot:
    there is no need for a full rebuild: a simple +make+ invocation
    will take the changes into account.
 
+ * When a pacjage listed in +FOO_DEPENDENCIES+ is rebuilt or removed,
+   the package +foo+ is not automatically rebuilt. For example, if a
+   package +bar+ is listed in +FOO_DEPENDENCIES+ with +FOO_DEPENDENCIES
+   = bar+ and the configuration of the +bar+ package is changed, the
+   configuration change would not result in a rebuild of package +foo+
+   automatically. In this scenario, you may need to either rebuild any
+   packages in your build which reference +bar+ in their +DEPENDENCIES+,
+   or perform a full rebuild to ensure any +bar+ dependent packages are
+   up to date.
+
 Generally speaking, when you're facing a build error and you're unsure
 of the potential consequences of the configuration changes you've
 made, do a full rebuild. If you get the same build error, then you are