move documentation of plugins to the .cxx files

It's mostly there already anyway, no need to duplicate it.

Change-Id: I5b066f90725a064fb0746e1411900e835e3f66c3

compilerplugins/clang/bodynotinblock.cxx

@@ -21,6 +21,14 @@ This is a compile check.
 
 Check for two statements that are both indented to look like a body of if/while/for
 but are not inside a compound statement and thus the second one is unrelated.
+
+For example:
+
+    if( a != 0 )
+        b = 2;
+        c = 3;
+
+Here either both statements should be inside {} or the second statement in indented wrong.
 */
 
 BodyNotInBlock::BodyNotInBlock( ASTContext& context )

compilerplugins/clang/plugin.cxx

@@ -15,6 +15,9 @@
 
 #include "pluginhandler.hxx"
 
+/*
+Base classes for plugin actions.
+*/
 namespace loplugin
 {
 

compilerplugins/clang/pluginhandler.cxx

@@ -16,6 +16,10 @@
 #include <stdio.h>
 #include <unistd.h>
 
+/*
+This source file manages all plugin actions. It is not necessary to modify this
+file when adding new actions.
+*/
 namespace loplugin
 {
 

compilerplugins/clang/postfixincrementfix.cxx

@@ -13,6 +13,12 @@
 #include <clang/AST/ASTContext.h>
 #include <clang/Basic/SourceManager.h>
 
+/*
+This is a rewriter.
+
+Change all postfix ++ operators of non-trivial types to prefix if possible.
+*/
+
 namespace loplugin
 {
 

compilerplugins/clang/sallogareas.cxx

@@ -22,7 +22,9 @@ namespace loplugin
 /*
 This is a compile check.
 
-Check that areas used in SAL_LOG/SAL_WARN are listed in sal/inc/sal/log-areas.dox .
+Check area used in SAL_INFO/SAL_WARN macros against the list in sal/inc/sal/log-areas.dox and
+report if the area is not listed there. The fix is either use a proper area or add it to the list
+if appropriate.
 */
 
 SalLogAreas::SalLogAreas( ASTContext& context )