[ruby/rubygems] Added another usage of pristine command

ref. rubygems/guides#388

ruby/rubygems@43f3768f09

Author: hsbt

lib/rubygems/commands/pristine_command.rb

@@ -88,6 +88,10 @@ def description # :nodoc:
 will revert them.  All extensions are rebuilt and all bin stubs for the gem
 are regenerated after checking for modifications.
 
+Rebuilding extensions also refreshes C-extension gems against updated system
+libraries (for example after OS or package upgrades) to avoid mismatches like
+outdated library version warnings.
+
 If the cached gem cannot be found it will be downloaded.
 
 If --no-extensions is provided pristine will not attempt to restore a gem