btrfs-progs: docs: add developer docs for internal APIs

Signed-off-by: David Sterba <dsterba@suse.com>

Documentation/dev/dev-internal-apis.rst

@@ -0,0 +1,36 @@
+Internal APIs
+=============
+
+There's some common functionality found in many places like help, parsing
+values, sorting, extensible arrays, etc. Not all places are unified and use old
+code implementing it manually. Below is list of usable APIs that should be spread
+and updated where it's still not. A need for new API might emerge from
+cleanups, then it should appear here.
+
+Option parsing
+--------------
+
+Files: common/help.h, common/parse-utils.h
+
+Global options need to be processed and consumed by `clean_args_no_options`,
+argument count by `check_argc_*`, `usage_*` for handling usage.
+
+Options are parsed by `getopt` or `getopt_long`. Individual values from options
+are recognized by `parse_*`, basic types and custom types are supported.
+
+TODO
+----
+
+Undocumented or incomplete APIs:
+
+* common/array.h
+* common/cpu-utils.h
+* common/device-utils.h
+* common/messages.h
+* common/open-utils.h
+* common/path-utils.h
+* common/sort-utils.h
+* common/string-table.h
+* common/string-table.h
+* common/task-utils.h
+* common/units.h

Documentation/index.rst

@@ -60,6 +60,7 @@ Welcome to BTRFS documentation!
    dev/On-disk-format
    dev/dev-send-stream
    dev/dev-json
+   dev/dev-internal-apis
    btrfs-ioctl
 
 .. toctree::