[Docs] document current and clarify .nvmrc fallback in help/README

- Help text for `nvm install`/`use`/`exec`/`run`/`which` now ends with
  "Uses .nvmrc if version is omitted; otherwise errors."
  so it's clear that omitting the version is not a free fallback.
- Help signatures for `use`/`exec`/`run` now show `[current | <version>]`
  to mirror `nvm which` and document that `current` is accepted.
- `nvm current` description now spells out that it resolves via `$PATH`
  and is not affected by `.nvmrc`.
- README: add `current` to the list of special aliases, with the same caveat.
  The `.nvmrc` section now states that `nvm use`/`install`/`which`
  exit with status 127 when neither a version nor an `.nvmrc` resolves,
  notes the (current) `exec`/`run` fallback as undefined behavior,
  and points readers at `current` for the explicit "active node" use case.

Refs #3755

test/fast/Running 'nvm --help' documents current and the .nvmrc fallback

@@ -0,0 +1,44 @@
+#!/bin/sh
+
+set -e
+
+die () { echo "$@" ; exit 1; }
+
+export NVM_DIR="$(cd ../.. && pwd)"
+
+: nvm.sh
+\. ../../nvm.sh
+
+\. ../common.sh
+
+HELP="$(nvm --help 2>&1)"
+
+# Signatures for use/exec/run advertise `current` (mirroring `nvm which`).
+for EXPECTED in \
+  'nvm use [current | <version>]' \
+  'nvm exec [current | <version>] [<command>]' \
+  'nvm run [current | <version>] [<args>]' \
+  'nvm which [current | <version>]' \
+; do
+  case "${HELP}" in
+    *"${EXPECTED}"*) ;;
+    *) die "nvm --help did not contain signature >${EXPECTED}<" ;;
+  esac
+done
+
+# The .nvmrc fallback is documented as conditional, not free.
+case "${HELP}" in
+  *'Uses .nvmrc if version is omitted; otherwise errors.'*) ;;
+  *) die "nvm --help did not document the .nvmrc fallback caveat" ;;
+esac
+
+# The stale, looser phrasing must be gone.
+case "${HELP}" in
+  *'if available and version is omitted'*) die "nvm --help still contains the old .nvmrc phrasing" ;;
+esac
+
+# `nvm current` is documented as resolving via \$PATH, not .nvmrc.
+case "${HELP}" in
+  *'Display the active node version (resolved via $PATH; not affected by .nvmrc).'*) ;;
+  *) die "nvm --help did not document that 'nvm current' resolves via \$PATH" ;;
+esac