log: Always enable; remove DISABLE_LOG

- Establish ERROR log level as "critical". Such errors are rare and will
  be valuable when users encounter unusual circumstances.
- Set -DMIN_LOG_LEVEL=3 for release-type builds

contrib/local.mk.example

@@ -31,9 +31,8 @@
 #
 # CMAKE_BUILD_TYPE := Debug
 
-# By default, nvim's log level is INFO (1) (unless CMAKE_BUILD_TYPE is
-# "Release", in which case logging is disabled).
-# The log level must be a number DEBUG (0), INFO (1), WARNING (2) or ERROR (3).
+# The default log level is 1 (INFO) (unless CMAKE_BUILD_TYPE is "Release").
+# Log levels: 0 (DEBUG), 1 (INFO), 2 (WARNING), 3 (ERROR)
 # CMAKE_EXTRA_FLAGS += -DMIN_LOG_LEVEL=1
 
 # By default, nvim uses bundled versions of its required third-party