Disclaimer about the default provider activation added to config

Fixes #16249

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16280)

apps/openssl-vms.cnf

@@ -60,6 +60,14 @@ default = default_sect
 # included fipsmodule.cnf.
 # fips = fips_sect
 
+# If no providers are activated explicitly, the default one is activated implicitly.
+# See man 7 OSSL_PROVIDER-default for more details.
+#
+# If you add a section explicitly activating any other provider(s), you most
+# probably need to explicitly activate the default provider, otherwise it
+# becomes unavailable in openssl.  As a consequence applications depending on
+# OpenSSL may not work correctly which could lead to significant system
+# problems including inability to remotely access the system.
 [default_sect]
 # activate = 1
 

apps/openssl.cnf

@@ -60,6 +60,14 @@ default = default_sect
 # included fipsmodule.cnf.
 # fips = fips_sect
 
+# If no providers are activated explicitly, the default one is activated implicitly.
+# See man 7 OSSL_PROVIDER-default for more details.
+#
+# If you add a section explicitly activating any other provider(s), you most
+# probably need to explicitly activate the default provider, otherwise it
+# becomes unavailable in openssl.  As a consequence applications depending on
+# OpenSSL may not work correctly which could lead to significant system
+# problems including inability to remotely access the system.
 [default_sect]
 # activate = 1
 

doc/man5/config.pod

@@ -273,6 +273,15 @@ significant.
 All parameters in the section as well as sub-sections are made
 available to the provider.
 
+=head3 Default provider and its activation
+
+If no providers are activated explicitly, the default one is activated implicitly.
+See L<OSSL_PROVIDER-default(7)> for more details.
+
+If you add a section explicitly activating any other provider(s),
+you most probably need to explicitly activate the default provider,
+otherwise it becomes unavailable in openssl. It may make the system remotely unavailable.
+
 =head2 EVP Configuration
 
 The name B<alg_section> in the initialization section names the section