Substitution config: doc strings for substitutions supplied by config content

[valentin-krasontovitsch]

Describe the bug At least some of the substitution pairs that may be used as part of a Substitution config have doc strings describing that particular substitution or config. When the substitution config is created using a config dict, there is python code that creates a substitution list, which is finally passed to the C layer to populate the C layer substitution list. ~~To that end~~ When alloced from config content, the function subst_config_add_subst_kw (in src/clib/lib/enkf/subst_config.cpp) is used, which adds the doc string Supplied by the user in the configuration file. any time it is called, regardless of passed key or value.

This is probably not the desired behaviour, as

• ~~the info actually came from a config dict, not a configuration file (but I might be wrong on this one, as the dict was probably parsed from a file!?)~~
• the doc string should probably (have the possibility to) be different for different key value pairs

I suggest that the config content path is adjusted to resemble the config dict path, and has descriptive doc strings for the substitution pairs.

Considering that we want to be rid of the config content path, this might not be the most urgent issue.

Related to milestone #3740