doc: per-user config file isn't hardcoded
doc: per-user config file isn't hardcoded

Document that the per-user config file respects the `$NPM_CONFIG_USERCONFIG` env var, defaulting to ~/.npmrc

PR-URL: https://github.com/npm/npm/pull/13493
Credit: da39a3ee5e6b4b0d3255bfef95601890afd80709@jasonkarns
Reviewed-By: da39a3ee5e6b4b0d3255bfef95601890afd80709@iarna

evocateur/npm,evocateur/npm,evocateur/npm