doc: emphasize trade-off between versionCheckHook and testers.testVersion

doc/build-helpers/testers.chapter.md

@@ -169,7 +169,7 @@ The build will fail if `shellcheck` finds any issues.
 
 Checks that the output from running a command contains the specified version string in it as a whole word.
 
-NOTE: In most cases, [`versionCheckHook`](#versioncheckhook) should be preferred, but this function is provided and documented here anyway. The motivation for adding either tests would be:
+NOTE: This is a check you add to `passthru.tests` which is mainly run by OfBorg, but not in Hydra. If you want a version check failure to block the build altogether, then [`versionCheckHook`](#versioncheckhook) is the tool you're looking for (and recommended for quick builds). The motivation for adding either of these checks would be:
 
 - Catch dynamic linking errors and such and missing environment variables that should be added by wrapping.
 - Probable protection against accidentally building the wrong version, for example when using an "old" hash in a fixed-output derivation.

doc/hooks/versionCheckHook.section.md

@@ -1,6 +1,8 @@
 # versionCheckHook {#versioncheckhook}
 
-This hook adds a `versionCheckPhase` to the [`preInstallCheckHooks`](#ssec-installCheck-phase) that runs the main program of the derivation with a `--help` or `--version` argument, and checks that the `${version}` string is found in that output. You use it like this:
+This hook adds a `versionCheckPhase` to the [`preInstallCheckHooks`](#ssec-installCheck-phase) that runs the main program of the derivation with a `--help` or `--version` argument, and checks that the `${version}` string is found in that output. If this check fails then the whole build will fail. _(A softer option is [`testers.testVersion`](#tester-testVersion).)_
+
+You use it like this:
 
 ```nix
 {
@@ -33,3 +35,5 @@ The variables that this phase control are:
 - `versionCheckProgramArg`: The argument that needs to be passed to `versionCheckProgram`. If undefined the hook tries first `--help` and then `--version`. Examples: `version`, `-V`, `-v`.
 - `preVersionCheck`: A hook to run before the check is done.
 - `postVersionCheck`: A hook to run after the check is done.
+
+This check assumes the executable is _hermetic_. If environment variables such as `PATH` or `HOME` are required for the program to function, then [`testers.testVersion`](#tester-testVersion) is currently the better alternative.