Speaking of "alias", I have always felt that it was suboptimal to
make users refer to "git help config" to find out about it.  I
wonder if "git help git" should be the first place users would look
for a help about them?

We have "GIT COMMANDS" section in "git help git" that says "We
divide GIt into porcelain and plumbing" and then have two
subsections there that list commands that belong to these two
categories.  Perhaps leaving some breadcrumbs to redirect them would
be a good start, something like this?

 Documentation/git.adoc | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git c/Documentation/git.adoc w/Documentation/git.adoc
index ce099e78b8..fb5b477eda 100644
--- c/Documentation/git.adoc
+++ w/Documentation/git.adoc

@@ -235,7 +235,10 @@ GIT COMMANDS
 ------------

 We divide Git into high level ("porcelain") commands and low level
-("plumbing") commands.
+("plumbing") commands.  For defining command aliases, see
+linkgit:gitconfig[1] and look for descriptions of `alias.*`.
+For installing custom "git" subcommands, see the description for
+the 'PATH' environment variable in this manual.

 High-level commands (porcelain)
 -------------------------------