[analyzer] Add check::BeginFunction to CheckerDocumentation checks. NFC.

This was an oversight from when I added BeginFunction support in r261293.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@276950 91177308-0d34-0410-b5e6-96231b3b80d8

lib/StaticAnalyzer/Checkers/CheckerDocumentation.cpp

@@ -45,6 +45,7 @@ class CheckerDocumentation : public Checker< check::PreStmt<ReturnStmt>,
                                        check::Location,
                                        check::Bind,
                                        check::DeadSymbols,
+                                       check::BeginFunction,
                                        check::EndFunction,
                                        check::EndAnalysis,
                                        check::EndOfTranslationUnit,