Documentation: migrate sub-process docs to header

Move the documentation for the sub-process API from a separate txt file
to its header file.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: jonathantanmy

Documentation/technical/api-sub-process.txt

@@ -6,12 +6,23 @@
 #include "run-command.h"
 
 /*
- * Generic implementation of background process infrastructure.
- * See: Documentation/technical/api-sub-process.txt
+ * The sub-process API makes it possible to run background sub-processes
+ * for the entire lifetime of a Git invocation. If Git needs to communicate
+ * with an external process multiple times, then this can reduces the process
+ * invocation overhead. Git and the sub-process communicate through stdin and
+ * stdout.
+ *
+ * The sub-processes are kept in a hashmap by command name and looked up
+ * via the subprocess_find_entry function.  If an existing instance can not
+ * be found then a new process should be created and started.  When the
+ * parent git command terminates, all sub-processes are also terminated.
+ *
+ * This API is based on the run-command API.
  */
 
  /* data structures */
 
+/* Members should not be accessed directly. */
 struct subprocess_entry {
 	struct hashmap_entry ent; /* must be the first member! */
 	const char *cmd;
@@ -20,21 +31,31 @@ struct subprocess_entry {
 
 /* subprocess functions */
 
+/* Function to test two subprocess hashmap entries for equality. */
 extern int cmd2process_cmp(const void *unused_cmp_data,
 			   const struct subprocess_entry *e1,
 			   const struct subprocess_entry *e2,
 			   const void *unused_keydata);
 
+/*
+ * User-supplied function to initialize the sub-process.  This is
+ * typically used to negotiate the interface version and capabilities.
+ */
 typedef int(*subprocess_start_fn)(struct subprocess_entry *entry);
+
+/* Start a subprocess and add it to the subprocess hashmap. */
 int subprocess_start(struct hashmap *hashmap, struct subprocess_entry *entry, const char *cmd,
 		subprocess_start_fn startfn);
 
+/* Kill a subprocess and remove it from the subprocess hashmap. */
 void subprocess_stop(struct hashmap *hashmap, struct subprocess_entry *entry);
 
+/* Find a subprocess in the subprocess hashmap. */
 struct subprocess_entry *subprocess_find_entry(struct hashmap *hashmap, const char *cmd);
 
 /* subprocess helper functions */
 
+/* Get the underlying `struct child_process` from a subprocess. */
 static inline struct child_process *subprocess_get_child_process(
 		struct subprocess_entry *entry)
 {