[llvm-extract] Update the help message for group extraction feature

Summary:
https://reviews.llvm.org/D60973 exposed the group extraction feature of
the BlockExtractor to llvm-extract.
However, the help message was not updated, so users might not be able to
know how to use this feature without looking into history/commits.

This patch just update the help message to show how to use this group
extraction feature.

Reviewers: qcolombet, volkan

Reviewed By: qcolombet

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66438

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@369438 91177308-0d34-0410-b5e6-96231b3b80d8

tools/llvm-extract/llvm-extract.cpp

@@ -74,8 +74,18 @@ static cl::list<std::string>
 
 // ExtractBlocks - The blocks to extract from the module.
 static cl::list<std::string> ExtractBlocks(
-    "bb", cl::desc("Specify <function, basic block> pairs to extract"),
-    cl::ZeroOrMore, cl::value_desc("function:bb"), cl::cat(ExtractCat));
+    "bb",
+    cl::desc(
+        "Specify <function, basic block1[;basic block2...]> pairs to extract.\n"
+        "Each pair will create a function.\n"
+        "If multiple basic blocks are specified in one pair,\n"
+        "the first block in the sequence should dominate the rest.\n"
+        "eg:\n"
+        "  --bb=f:bb1;bb2 will extract one function with both bb1 and bb2;\n"
+        "  --bb=f:bb1 --bb=f:bb2 will extract two functions, one with bb1, one "
+        "with bb2."),
+    cl::ZeroOrMore, cl::value_desc("function:bb1[;bb2...]"),
+    cl::cat(ExtractCat));
 
 // ExtractAlias - The alias to extract from the module.
 static cl::list<std::string>