ancestor and descendant ~ clarification needed

Hi,

In Documentation/user-manual.txt:

   In the following, we say that commit X is "reachable" from commit Y
   if commit X is an ancestor of commit Y.  Equivalently, you could say
   that Y is a descendant of X, or that there is a chain of parents
   leading from commit Y to commit X.
[...]
   However, if the current branch is a descendant of the other--so every
   commit present in the one is already contained in the other--then Git
   just performs a "fast-forward"; the head of the current branch is
   moved forward to point at the head of the merged-in branch, without
   any new commits being created.

I'm a Git newbie. According to my understanding, the "descendant" in the second paragraph above should be "ancestor". I attempt to represent my understanding using the following diagram (please see it in a monospaced font):

................................................

         o--o--o <-- Branch A
        /
 o--o--o <-- master

................................................

"master" is the current branch, and (as I understand it) it is an ancestor of "Branch A", because there is a chain of parents leading from "Branch A" to master. So "Branch A" (i.e., "the other" branch, or the "merged-in" branch) is a descendant of master. I even set up a test repository and attempted to test the above diagram with "git merge-base --is-ancestor" (and "echo $?"), but it seems to me that the master branch is *not* a descendant of "Branch A".

I hope you can understand my words here (English is not my native language). Can anyone point me in the right direction (what am I missing)?