The Manual about the MAIL environment is misleading

[ikrabbe . ask]

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: x86_64-pc-linux-gnu-gcc
Compilation CFLAGS:  -DPROGRAM='bash' -DCONF_HOSTTYPE='x86_64' 
-DCONF_OSTYPE='linux-gnu' -DCONF_MACHTYPE='x86_64-pc-linux-gnu' 
-DCONF_VENDOR='pc' -DLOCALEDIR='/usr/share/locale' -DPACKAGE='bash' -DSHELL 
-DHAVE_CONFIG_H   -I. -I./include -I. -I./include -I./lib  
-DDEFAULT_PATH_VALUE='/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin'
 -DSTANDARD_UTILS_PATH='/bin:/usr/bin:/sbin:/usr/sbin' 
-DSYS_BASHRC='/etc/bash/bashrc' -DSYS_BASH_LOGOUT='/etc/bash/bash_logout' 
-DNON_INTERACTIVE_LOGIN_SHELLS -DSSH_SOURCE_BASHRC -DUSE_MKTEMP -DUSE_MKSTEMP 
-O2 -pipe
uname output: Linux laptop 4.6.1jun2016 #24 SMP Sat Jun 18 09:56:03 CEST 2016 
x86_64 Intel(R) Core(TM) i7-4700MQ CPU @ 2.40GHz GenuineIntel GNU/Linux
Machine Type: x86_64-pc-linux-gnu

Bash Version: 4.3
Patch Level: 42
Release Status: release

Description:
        In a bash session where the MAIL variable is not explicitly set, the 
MAIL variable might be auto-set through a compile time definition of 
DEFAULT_MAIL_DIRECTORY. There is no method (no method I know of) to find about 
about these compile time definitions and / or configuration flags.
        To find out about this bevaiour I needed to read the source code and 
finally found this line in CHANGES:

        hhh. Improved the mail checking code so it won't check (and possibly 
cause an
             NFS file system mount) until MAILPATH or MAIL is given a value -- 
there
             is no default if DEFAULT_MAIL_DIRECTORY is not defined at compile 
time.
        

Repeat-By:
        Install bash and remove all occurences of MAIL= or similar from the 
several profile and rc files.

Fix:
        Please describe the dependency of the MAIL variables on the manual 
page. If you manage to do so, add the value of DEFAULT_MAIL_DIRECTORY and it's 
effects in the generated manual pages.