docs/manual: rephrase end of vendoring section

The current phrasing is not entirely clear: one could read it as if
Buildroot will detect if there's an update available in one of the
dependencies, which is quite the reverse of what we do. Rephrase the
sentence in a way that hopefully makes it clearer that we're just making
a hash of the dependencies.

While we're at it, also extend the sentence about offline builds a
little.

Suggested-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Arnout Vandecappelle <arnout@mind.be>

docs/manual/adding-packages-cargo.adoc

@@ -93,7 +93,9 @@ takes care of downloading such dependencies as part of the download
 step of packages that use the +cargo-package+ infrastructure. Such
 dependencies are then kept together with the package source code in
 the tarball cached in Buildroot's +DL_DIR+, and therefore the hash of
-the package's tarball includes such dependencies.
-
-This mechanism ensures that any change in the dependencies will be
-detected, and allows the build to be performed completely offline.
+the package's tarball doesn't only cover the source of the package
+itself, but also covers the sources of the dependencies. Thus, a change
+injected into one of the dependencies will also be discovered by the
+hash check. In addition,  this mechanism allows the build to be
+performed completely offline since cargo will not do any downloads
+during the build. This mechanism is called vendoring the dependencies.