revision selection: add clarity to explanation of commit parents

Expand slightly on precisely which commits are the first and
second parents of a merge commit.

Signed-off-by: Robert P. J. Day <[email protected]>

Author: robertpjday

book/07-git-tools/sections/revision-selection.asc

@@ -227,9 +227,8 @@ $ git show "HEAD^"   # OK
 
 ====
 
-You can also specify a number after the `^` – for example, `d921970^2` means ``the second parent of d921970.''
-This syntax is useful only for merge commits, which have more than one parent.
-The first parent is the branch you were on when you merged, and the second is the commit on the branch that you merged in:
+You can also specify a number after the `^` to identify _which_ parent you want; for example, `d921970^2` means ``the second parent of d921970.''
+This syntax is useful only for merge commits, which have more than one parent -- the _first_ parent of a merge commit is from the branch you were on when you merged (frequently `master`), while the _second_ parent of a merge commit is from the branch that was merged (say, `topic`):
 
 [source,console]
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