Describe dpm_module_execute as the entry point, not the whole surface

A module's reported version is consumed: a calling module reads it
through dpm_module_info_of and decides from it whether to issue a
command. Calling dpm_module_execute the module's entire functional
surface contradicted that.

dpm_module_execute is the only entry through which a module performs
work. dpm_module_version and dpm_module_description are what it reports
about itself, read at load and served to consumers, and the contract
page in the public header now says so.

docs/DESIGN.md

@@ -103,7 +103,7 @@ Returns a human-readable description of the most recent failure recorded on this
 A module is one .so in the module directory. It includes `<dpm/core.h>`, links `-ldpm-core`, and exports the following reserved symbols as extern "C". Returned strings are static or module-owned, non-NULL, and valid for the lifetime of the loaded module; the library and consumers never free them.
 
 **`int dpm_module_execute(dpm_ctx* ctx, const char* command, int argc, char** argv)`**
-The module's command entry point, and its entire functional surface. `ctx` is the host context that dispatched the call — the module reaches every service (`dpm_log`, `dpm_config_get`, `dpm_module_path`, ...) through it, and reaches peer modules through it as well. `command` is the subcommand name (equal to argv[0] when argc > 0); argc/argv are the remaining CLI-style arguments. NULL or empty `command` must behave as the module's help command. Returns 0 on success, nonzero on failure. It must be callable immediately after load with no other setup.
+The module's command entry point, and the only entry through which it performs work. `ctx` is the host context that dispatched the call — the module reaches every service (`dpm_log`, `dpm_config_get`, `dpm_module_path`, ...) through it, and reaches peer modules through it as well. `command` is the subcommand name (equal to argv[0] when argc > 0); argc/argv are the remaining CLI-style arguments. NULL or empty `command` must behave as the module's help command. Returns 0 on success, nonzero on failure. It must be callable immediately after load with no other setup.
 
 **`const char* dpm_module_version(void)`**
 Returns the module's own version as an X.Y.Z string. Must be constant for the life of the module. This is the value the library reports to consumers, and the value they judge compatibility against.

docs/MODULES.md

@@ -7,7 +7,7 @@ A DPM module is one shared object in the module directory. libdpm-core.so loads
 A module includes `<dpm/core.h>`, links `-ldpm-core`, and exports the following symbols as extern "C". All returned strings must be non-NULL, static or module-owned, and valid for the lifetime of the loaded module; callers never free them.
 
 **`int dpm_module_execute(dpm_ctx* ctx, const char* command, int argc, char** argv)`**
-The command entry point, and the module's entire functional surface. `ctx` is the host context that dispatched the call; the module reaches every service (`dpm_log`, `dpm_config_get`, `dpm_module_path`, ...) through it, and reaches peer modules through it as well. `command` is the subcommand name, equal to argv[0] when argc > 0. NULL or empty `command` must behave as the module's help command. Returns 0 on success, nonzero on failure. It must be callable immediately after load with no other setup.
+The command entry point, and the only entry through which the module performs work. `ctx` is the host context that dispatched the call; the module reaches every service (`dpm_log`, `dpm_config_get`, `dpm_module_path`, ...) through it, and reaches peer modules through it as well. `command` is the subcommand name, equal to argv[0] when argc > 0. NULL or empty `command` must behave as the module's help command. Returns 0 on success, nonzero on failure. It must be callable immediately after load with no other setup.
 
 **`const char* dpm_module_version(void)`**
 The module's own version as an X.Y.Z string. libdpm-core.so reports this value to consumers, and each consumer decides for itself whether the version suits it.