doc: clarify env vars to set prior Modules initialization

NEWS.rst

@@ -173,6 +173,10 @@ Modules 5.1.0 (not yet released)
   page to provide for each Modules environment variable the name of the
   configuration option that could be used to set the variable with the
   :subcmd:`config` sub-command.
+* Doc: clarify in :ref:`module(1)` man page that
+  :envvar:`MODULES_SET_SHELL_STARTUP`, :envvar:`MODULES_SHELLS_WITH_KSH_FPATH`
+  and :envvar:`MODULES_WA_277` should be set prior Modules initialization to
+  get taken into account.
 
 .. _Code of conduct: https://github.com/cea-hpc/modules/blob/master/CODE_OF_CONDUCT.md
 .. _codespell: https://github.com/codespell-project/codespell

doc/source/module.rst

@@ -2915,6 +2915,11 @@ ENVIRONMENT
  :mconfig:`set_shell_startup` configuration option. It can be defined with the
  :subcmd:`config` sub-command.
 
+ To enable shell startup file, :envvar:`MODULES_SET_SHELL_STARTUP` should be
+ set to ``1`` prior Modules initialization or the :mconfig:`set_shell_startup`
+ configuration option should be set to ``1`` in the :file:`initrc`
+ configuration file.
+
  .. only:: html
 
     .. versionadded:: 4.3
@@ -2934,6 +2939,12 @@ ENVIRONMENT
  :mconfig:`shells_with_ksh_fpath` configuration option. It can be defined with
  the :subcmd:`config` sub-command.
 
+ To enable the setup of :envvar:`FPATH` for some shells,
+ :envvar:`MODULES_SHELLS_WITH_KSH_FPATH` should be set to the list of these
+ shells prior Modules initialization or the :mconfig:`shells_with_ksh_fpath`
+ configuration option should be set to the list of these shells in the
+ :file:`initrc` configuration file.
+
  .. only:: html
 
     .. versionadded:: 4.7
@@ -3137,6 +3148,10 @@ ENVIRONMENT
  :mconfig:`wa_277` configuration option. It can be defined with the
  :subcmd:`config` sub-command.
 
+ To enable this workaround, :envvar:`MODULES_WA_277` should be set to ``1``
+ prior Modules initialization or the :mconfig:`wa_277` configuration option
+ should be set to ``1`` in the :file:`initrc` configuration file.
+
  .. only:: html
 
     .. versionadded:: 4.3