config updated to improve setting values, writing changes and test coverage

internal/config/README.md

@@ -116,10 +116,12 @@ Nothing in `section.go` needs changing.
 - `conf.Map()` (nested) and `conf.Flatten()` (dotted keys): the whole
   configuration as a map.
 - `conf.String()`, `conf.JSON()`, `conf.YAML()` and the matching `Print*`
-  methods: the configuration as text.
+  methods: the configuration as text, with keys sorted.
 - `StoreValue(&conf, key, value)` / `PurgeValue(&conf, key)`: set or remove a
   key in the config file Viper loaded. Used by `config set` and
-  `config set --unset`. `StoreValue` validates the value before writing it.
+  `config set --unset`. `StoreValue` validates the value before writing it,
+  and stores it as the field's own type. Only the file's own settings are
+  written back; defaults and environment variables never are.
 - `LoadDefaultConfig`, `LoadToViper`, `LoadFromViper`: move values between
   the struct defaults, Viper and a `JobSearchConfig`.