doc: Update stdenv.chapter.md (NixOS#274706)

Extended genericBuild description

doc/stdenv/stdenv.chapter.md

@@ -591,7 +591,7 @@ See also the section about [`passthru.tests`](#var-meta-tests).
 
 `stdenv.mkDerivation` sets the Nix [derivation](https://nixos.org/manual/nix/stable/expressions/derivations.html#derivations)'s builder to a script that loads the stdenv `setup.sh` bash library and calls `genericBuild`. Most packaging functions rely on this default builder.
 
-This generic command invokes a number of *phases*. Package builds are split into phases to make it easier to override specific parts of the build (e.g., unpacking the sources or installing the binaries).
+This generic command either invokes a script at *buildCommandPath*, or a *buildCommand*, or a number of *phases*. Package builds are split into phases to make it easier to override specific parts of the build (e.g., unpacking the sources or installing the binaries).
 
 Each phase can be overridden in its entirety either by setting the environment variable `namePhase` to a string containing some shell commands to be executed, or by redefining the shell function `namePhase`. The former is convenient to override a phase from the derivation, while the latter is convenient from a build script. However, typically one only wants to *add* some commands to a phase, e.g. by defining `postInstall` or `preFixup`, as skipping some of the default actions may have unexpected consequences. The default script for each phase is defined in the file `pkgs/stdenv/generic/setup.sh`.