[master] b63b5d2ce Revert "Retire also the reference to VSM_NOPID in docs"

[Dridi Boukelmoune]

commit b63b5d2ce94c31d2a0ddb350ffdca4adcc5542a0
Author: Dridi Boukelmoune <dridi.boukelmoune at gmail.com>
Date:   Sat May 25 08:01:54 2024 +0200

    Revert "Retire also the reference to VSM_NOPID in docs"
    
    This reverts commit a3593245f5a9f149da4890dbc678dcf9142929dd.

diff --git a/doc/sphinx/reference/vsm.rst b/doc/sphinx/reference/vsm.rst
index ad2da1740..9d09ce77a 100644
--- a/doc/sphinx/reference/vsm.rst
+++ b/doc/sphinx/reference/vsm.rst
@@ -102,3 +102,22 @@ a chance to discover the deallocation.
 
 The include file <vapi/vsm.h> provides the supported API for accessing
 VSM files.
+
+VSM and Containers
+------------------
+
+The varnish way works great with single purpose containers. By sharing
+the varnish working directory read-only, vsm readers can be run in
+containers separate from those running varnishd instances on the same
+host.
+
+When running varnishd and vsm readers in the same process namespace,
+pid information can be used by vsm readers to determine if varnishd
+processes are alive.
+
+But, when running varnishd and vsm readers in different containers,
+the pid information has no relevance and may even be ambiguous across
+name spaces.
+
+Thus, with such setups, the environment variable VSM_NOPID needs to be
+set for vsm readers to disable use of pid information.