Kconfig: improve KALLSYMS_ALL documentation

Dumb users like myself are not able to grasp from the existing KALLSYMS_ALL
documentation that this option is not what they need. Improve the help
message and make it clearer that KALLSYMS is enough in the majority of
use cases, and KALLSYMS_ALL should really be used very rarely.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>

init/Kconfig

@@ -968,12 +968,18 @@ config KALLSYMS_ALL
 	bool "Include all symbols in kallsyms"
 	depends on DEBUG_KERNEL && KALLSYMS
 	help
-	   Normally kallsyms only contains the symbols of functions, for nicer
-	   OOPS messages.  Some debuggers can use kallsyms for other
-	   symbols too: say Y here to include all symbols, if you need them 
-	   and you don't care about adding 300k to the size of your kernel.
+	   Normally kallsyms only contains the symbols of functions for nicer
+	   OOPS messages and backtraces (i.e., symbols from the text and inittext
+	   sections). This is sufficient for most cases. And only in very rare
+	   cases (e.g., when a debugger is used) all symbols are required (e.g.,
+	   names of variables from the data sections, etc).
 
-	   Say N.
+	   This option makes sure that all symbols are loaded into the kernel
+	   image (i.e., symbols from all sections) in cost of increased kernel
+	   size (depending on the kernel configuration, it may be 300KiB or
+	   something like this).
+
+	   Say N unless you really need all symbols.
 
 config KALLSYMS_EXTRA_PASS
 	bool "Do an extra kallsyms pass"