btrfs-progs: docs: add path-utils.h API docs

[ ci skip ]

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

Documentation/dev/dev-internal-apis.rst

@@ -34,6 +34,17 @@ Printing options is done by `pretty_size_mode` which takes the value and option
 mode. Default mode is human readable, the macros defining the modes are from
 `UNITS_*` namespace.
 
+File path handling
+------------------
+
+Files: :file:`common/path-utils.h`
+
+The paths on Linux can be at most PATH_MAX, which is 4096 (:command:`getconf`).
+For easier handling use a local variable like :code:`char path[PATH_MAX] = { 0 };`
+and for concatenation helpers :code:`path_cat_out()` or
+:code:`path_cat_out3()` and check the error values for overflows. There are
+helpers to check file type :code:`path_is_*()`.
+
 TODO
 ----
 
@@ -44,8 +55,6 @@ Undocumented or incomplete APIs:
 * 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