Add documentation about the multilib postfix and libdir

Fixes #16244

Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/16281)

CHANGES.md

@@ -30,6 +30,16 @@ breaking changes, and mappings for the large list of deprecated functions.
 
 ### Changes between 1.1.1 and 3.0 [xx XXX xxxx]
 
+ * On build targets where the multilib postfix is set in the build
+   configuration the libdir directory was changing based on whether
+   the lib directory with the multilib postfix exists on the system
+   or not. This unpredictable behavior was removed and eventual
+   multilib postfix is now always added to the default libdir. Use
+   `--libdir=lib` to override the libdir if adding the postfix is
+   undesirable.
+
+   *Jan Lána*
+
  * The ERR_GET_FUNC() function was removed.  With the loss of meaningful
    function codes, this function can only cause problems for calling
    applications.

INSTALL.md

@@ -350,9 +350,13 @@ Directories
 
 The name of the directory under the top of the installation directory tree
 (see the `--prefix` option) where libraries will be installed.  By default
-this is `lib/`. Note that on Windows only static libraries (`*.lib`) will
+this is `lib`. Note that on Windows only static libraries (`*.lib`) will
 be stored in this location. Shared libraries (`*.dll`) will always be
-installed to the `bin/` directory.
+installed to the `bin` directory.
+
+Some build targets have a multilib postfix set in the build configuration.
+For these targets the default libdir is `lib<multilib-postfix>`. Please use
+`--libdir=lib` to override the libdir if adding the postfix is undesirable.
 
 ### openssldir