doc: add explanations of OB_RANDOM_SEED environment variable

TODO: please comment them in when OpenBabel 4.0.0 is released

doc/obabel.1

@@ -520,6 +520,12 @@ For further specific information and options, use
 .Fl H Ar format-type ,
 e.g.,
 .Fl Hcml
+\"\" TODO comment in when OB 4.0.0 is released
+\".Sh ENVIRONMENT
+\".Bl -tag -width flag
+\".It Ev OB_RANDOM_SEED
+\"Seed for pseudo random number generator.  If unset or null, random seed is used.
+\".El
 .Sh EXAMPLES
 Standard conversion:
 .Dl "obabel ethanol.xyz \-Oethanol.pdb"

doc/obdistgen.1

@@ -9,6 +9,12 @@
 .Ar filename
 .Sh DESCRIPTION
 Generates SDFs with distance geometry.
+\"\" TODO comment in when OB 4.0.0 is released
+\".Sh ENVIRONMENT
+\".Bl -tag -width flag
+\".It Ev OB_RANDOM_SEED
+\"Seed for pseudo random number generator.  If unset or null, random seed is used.
+\".El
 .Sh EXAMPLES
 .Dl "obdistgen molecule.smi > molecule.sdf"
 .Pp

doc/obgen.1

@@ -24,6 +24,12 @@ will give all options including the available forcefields.
 .It Fl ff Ar forcefield
 Select the forcefield
 .El
+\"\" TODO comment in when OB 4.0.0 is released
+\".Sh ENVIRONMENT
+\".Bl -tag -width flag
+\".It Ev OB_RANDOM_SEED
+\"Seed for pseudo random number generator.  If unset or null, random seed is used.
+\".El
 .Sh EXAMPLES
 .Pp
 View the possible options, including available forcefields:

doc/obrotamer.1

@@ -17,6 +17,12 @@ rotamers are not conformers -- that is,
 does not perform geometry optimization after generating the rotamer
 structure. The obminimize tool can do geometry optimization using
 molecular mechanics.
+\"\" TODO comment in when OB 4.0.0 is released
+\".Sh ENVIRONMENT
+\".Bl -tag -width flag
+\".It Ev OB_RANDOM_SEED
+\"Seed for pseudo random number generator.  If unset or null, random seed is used.
+\".El
 .Sh EXAMPLES
 .Dl "obrotamer baseconformer.sdf > rotamer1.sdf"
 .Pp