doc: Clarify some of guix-daemon's GC options.

Reported by Pierre Neidhardt <ambrevar@gmail.com>.

* doc/guix.texi (Invoking guix-daemon): Fix an incorrect statement
regarding --gc-keep-outputs.  Add a little to the description of this
option and --gc-keep-derivations.

1 files changed, 9 insertions, 8 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index 2df4064524..19c9813f6a 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -1375,8 +1375,8 @@ derivations.
 @cindex garbage collector roots
 When set to ``yes'', the GC will keep the outputs of any live derivation
 available in the store---the @code{.drv} files.  The default is ``no'',
-meaning that derivation outputs are kept only if they are GC roots.
-@xref{Invoking guix gc}, for more on GC roots.
+meaning that derivation outputs are kept only if they are reachable from a GC
+root.  @xref{Invoking guix gc}, for more on GC roots.
 
 @item --gc-keep-derivations[=yes|no]
 Tell whether the garbage collector (GC) must keep derivations
@@ -1387,12 +1387,13 @@ derivations---i.e., @code{.drv} files---as long as at least one of their
 outputs is live.  This allows users to keep track of the origins of
 items in their store.  Setting it to ``no'' saves a bit of disk space.
 
-Note that when both @code{--gc-keep-derivations} and
-@code{--gc-keep-outputs} are used, the effect is to keep all the build
-prerequisites (the sources, compiler, libraries, and other build-time
-tools) of live objects in the store, regardless of whether these
-prerequisites are live.  This is convenient for developers since it
-saves rebuilds or downloads.
+In this way, setting @code{--gc-keep-derivations} to ``yes'' causes liveness
+to flow from outputs to derivations, and setting @code{--gc-keep-outputs} to
+``yes'' causes liveness to flow from derivations to outputs.  When both are
+set to ``yes'', the effect is to keep all the build prerequisites (the
+sources, compiler, libraries, and other build-time tools) of live objects in
+the store, regardless of whether these prerequisites are reachable from a GC
+root.  This is convenient for developers since it saves rebuilds or downloads.
 
 @item --impersonate-linux-2.6
 On Linux-based systems, impersonate Linux 2.6.  This means that the