trivial: doc: document missing value 2 for randomize-va-space

The documentation for /proc/sys/kernel/* does not mention the possible
value 2 for randomize-va-space yet.  While being there, doing some
reformatting, fixing grammar problems and clarifying the correlations
between randomize-va-space, kernel parameter "norandmaps" and the
CONFIG_COMPAT_BRK option.

Signed-off-by: Horst Schirmeier <horst@schirmeier.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

Documentation/sysctl/kernel.txt

@@ -319,25 +319,29 @@ This option can be used to select the type of process address
 space randomization that is used in the system, for architectures
 that support this feature.
 
-0 - Turn the process address space randomization off by default.
+0 - Turn the process address space randomization off.  This is the
+    default for architectures that do not support this feature anyways,
+    and kernels that are booted with the "norandmaps" parameter.
 
 1 - Make the addresses of mmap base, stack and VDSO page randomized.
     This, among other things, implies that shared libraries will be
-    loaded to random addresses. Also for PIE-linked binaries, the location
-    of code start is randomized.
+    loaded to random addresses.  Also for PIE-linked binaries, the
+    location of code start is randomized.  This is the default if the
+    CONFIG_COMPAT_BRK option is enabled.
 
-    With heap randomization, the situation is a little bit more
-    complicated.
-    There a few legacy applications out there (such as some ancient
+2 - Additionally enable heap randomization.  This is the default if
+    CONFIG_COMPAT_BRK is disabled.
+
+    There are a few legacy applications out there (such as some ancient
     versions of libc.so.5 from 1996) that assume that brk area starts
-    just after the end of the code+bss. These applications break when
-    start of the brk area is randomized. There are however no known
+    just after the end of the code+bss.  These applications break when
+    start of the brk area is randomized.  There are however no known
     non-legacy applications that would be broken this way, so for most
-    systems it is safe to choose full randomization. However there is
-    a CONFIG_COMPAT_BRK option for systems with ancient and/or broken
-    binaries, that makes heap non-randomized, but keeps all other
-    parts of process address space randomized if randomize_va_space
-    sysctl is turned on.
+    systems it is safe to choose full randomization.
+
+    Systems with ancient and/or broken binaries should be configured
+    with CONFIG_COMPAT_BRK enabled, which excludes the heap from process
+    address space randomization.
 
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