revwalk: reword the push text

As it seems it's not quite clear what it means to push a commit, try to
be more explicit about adding a new root and that we may not see this
commit if it is hidden.

include/git2/revwalk.h

@@ -90,15 +90,17 @@ GIT_EXTERN(int) git_revwalk_new(git_revwalk **out, git_repository *repo);
 GIT_EXTERN(void) git_revwalk_reset(git_revwalk *walker);
 
 /**
- * Mark a commit to start traversal from.
+ * Add a new root for the traversal
  *
- * The given OID must belong to a committish on the walked
- * repository.
+ * The pushed commit will be marked as one of the roots from which to
+ * start the walk. This commit may not be walked if it or a child is
+ * hidden.
+ *
+ * At least one commit must be pushed onto the walker before a walk
+ * can be started.
  *
- * The given commit will be used as one of the roots
- * when starting the revision walk. At least one commit
- * must be pushed onto the walker before a walk can
- * be started.
+ * The given id must belong to a committish on the walked
+ * repository.
  *
  * @param walk the walker being used for the traversal.
  * @param id the oid of the commit to start from.
@@ -135,7 +137,7 @@ GIT_EXTERN(int) git_revwalk_push_head(git_revwalk *walk);
 /**
  * Mark a commit (and its ancestors) uninteresting for the output.
  *
- * The given OID must belong to a committish on the walked
+ * The given id must belong to a committish on the walked
  * repository.
  *
  * The resolved commit and all its parents will be hidden from the