Fix BUILD_CAFFE2 if FBGEMM and NNPACK are not built (#45610)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45610

Also add to the usual documentation places that this option exists.

Test Plan: Imported from OSS

Reviewed By: gmagogsfm

Differential Revision: D24058199

Pulled By: suo

fbshipit-source-id: 81574fbd042f47587e2c7820c726fac0f68af2a7
diff --git a/setup.py b/setup.py
index cb6c054..b2270db 100644
--- a/setup.py
+++ b/setup.py
@@ -61,6 +61,9 @@
 #   BUILD_CAFFE2_OPS=0
 #     disable Caffe2 operators build
 #
+#   BUILD_CAFFE2=0
+#     disable Caffe2 build
+#
 #   USE_IBVERBS
 #     toggle features related to distributed support
 #