MXS-1405: Move externcmd documentation to header

The function documentation is more useful in the public header.

server/core/maxscale/externcmd.h

@@ -30,11 +30,68 @@ typedef struct extern_cmd_t
 } EXTERNCMD;
 
 char* externcmd_extract_command(const char* argstr);
-EXTERNCMD* externcmd_allocate(char* argstr);
+
+/**
+ * Allocate a new external command.
+ *
+ * The name and parameters are copied into the external command structure so
+ * the original memory can be freed if needed.
+ *
+ * @param command Command to execute with the parameters
+ * @param timeout Command timeout in seconds
+ *
+ * @return Pointer to new external command struct or NULL if an error occurred
+ */
+EXTERNCMD* externcmd_allocate(char* argstr, uint32_t timeout);
+
+
+/**
+ * Free a previously allocated external command
+ *
+ * @param cmd Command to free
+ */
 void externcmd_free(EXTERNCMD* cmd);
+
+/**
+ * Execute a command
+ *
+ * @param cmd Command to execute
+ *
+ * @return The return value of the executed command or -1 on error
+ */
 int externcmd_execute(EXTERNCMD* cmd);
+
+/**
+ * Substitute all occurrences of @c match with @c replace in the arguments for @c cmd
+ *
+ * @param cmd External command
+ * @param match Match string
+ * @param replace Replacement string
+ *
+ * @return True if replacement was successful, false on error
+ */
 bool externcmd_substitute_arg(EXTERNCMD* cmd, const char* re, const char* replace);
+
+/**
+ * Check if a command can be executed
+ *
+ * Checks if the file being executed exists and if the current user has execution
+ * permissions on the file.
+ *
+ * @param argstr Command to check, can contain arguments for the command
+ *
+ * @return True if the file was found and the use has execution permissions to it
+ */
 bool externcmd_can_execute(const char* argstr);
+
+/**
+ * Simple matching of string and command
+ *
+ * @param cmd Command where the match is searched from
+ * @param match String to search for
+ *
+ * @return True if the string matched
+ */
 bool externcmd_matches(const EXTERNCMD* cmd, const char* match);
 
 MXS_END_DECLS