docs: document Makefile targets

Targets intended for interactive use are documented via "make help".
Targets intended for internal and CI use are documented just with
comments.

Author: cfm

securedrop-protocol/protocol-minimal/Makefile

@@ -7,39 +7,48 @@ HAX_TARGETS += +securedrop_protocol_minimal::primitives::x25519::typed
 REPO_ROOT = $(shell git rev-parse --show-toplevel)
 PROOF_DIR = proofs/fstar/extraction
 
+.DEFAULT_GOAL := help
+
 .PHONY: clean
-clean:
+clean:  ## Clean the Cargo and F* caches.
 	cargo clean
 	cargo cache -a
 	rm -rf $(REPO_ROOT)/.fstar-cache
 	rm -f $(PROOF_DIR)/.depend
 	rm -f $(PROOF_DIR)/hax.fst.config.json
 
 .PHONY: compat
-compat: compat-fstar
+compat: compat-fstar  ## Check that hax depedencies are at the right versions.
 
 # Keep in sync with ".github/workflows/hax.yml":
 FSTAR_EXPECTED_VERSION = F* 2025.12.15
 FSTAR_ACTUAL_VERSION   = $(shell fstar.exe --version 2>/dev/null | head -1)
 .PHONY: compat-fstar
-compat-fstar:
+compat-fstar:  # Internal: Check F* version.
 	@[ "$(FSTAR_ACTUAL_VERSION)" = "$(FSTAR_EXPECTED_VERSION)" ] || \
 	  { echo "Error: expected fstar.exe version '$(FSTAR_EXPECTED_VERSION)', got '$(FSTAR_ACTUAL_VERSION)'"; exit 1; }
 
 .PHONY: extract
-extract:
+extract:  # Internal/CI: Run hax extraction.
 	cargo hax into -i '$(HAX_TARGETS)' fstar
 
 .PHONY: hax
-hax: compat extract verify
+hax: compat extract verify  ## Run hax extraction, then type-check and verify extracted proofs.
 
 .PHONY: verify
-verify:
+verify:  # Internal/CI: Type-check and verify extracted proofs.
 	# Type-check (fail fast):
 	OTHERFLAGS="--lax" $(MAKE) -C $(PROOF_DIR)
 	# Verify:
 	$(MAKE) -C $(PROOF_DIR)
 
 .PHONY: hax-lib-version
-hax-lib-version:
-	@cargo pkgid hax-lib 2>/dev/null | sed 's/.*@//'
+hax-lib-version:  # Internal: Query Cargo for the hax-lib version.
+	@cargo pkgid hax-lib 2>/dev/null | sed 's/.*@//'
+
+.PHONY: help
+help: ## Print this message.
+	@printf "Subcommands:\n\n"
+	@perl -F':.*##\s+' -lanE '$$F[1] and say "\033[36m$$F[0]\033[0m : $$F[1]"' $(MAKEFILE_LIST) \
+		| sort \
+		| column -s ':' -t