Revert of Remove the 'gyp_config' concept from MB.

tools/mb/docs/user_guide.md

Index: tools/mb/docs/user_guide.md
diff --git a/tools/mb/docs/user_guide.md b/tools/mb/docs/user_guide.md
index 6185d9ef25835d89556ae6c8a29c88626ec3d708..e1aba6fd6f02b5ddb582f1e92f9bb25a1903f77c 100644
--- a/tools/mb/docs/user_guide.md
+++ b/tools/mb/docs/user_guide.md
@@ -172,6 +172,8 @@
 Each mixin value is itself a dictionary that contains one or more of the
 following keys:
 
+  * `gyp_configs`: a list of the configurations to build, e.g.,
+    ['Release', 'Release_x64'].
   * `gyp_crosscompile`: a boolean; if true, GYP_CROSSCOMPILE=1 is set in
     the environment and passed to GYP.
   * `gyp_defines`: a string containing a list of GYP_DEFINES.
@@ -182,8 +184,8 @@
 
 When `mb gen` or `mb analyze` executes, it takes a config name, looks it
 up in the 'configs' dict, and then does a left-to-right expansion of the
-mixins; gyp_defines and gn_args values are concatenated, and the type values
-overrides each other.
+mixins; gyp_defines and gn_args values are concatenated, and type and
+gyp_configs values override each other.
 
 For example, if you had:
 
@@ -203,6 +205,7 @@
     },
     'gn': {'type': 'gn'},
     'gyp_release': {
+      'gyp_config': 'Release'
       'mixins': ['release'],
       'type': 'gyp',
     },