docs(css): add ffi docs

Author: TtTRz

float-pigment-css/cbindgen/cbindgen.toml

@@ -34,7 +34,7 @@ after_includes = ""
 braces = "SameLine"
 line_length = 100
 tab_width = 2
-documentation = false
+documentation = true
 documentation_style = "auto"
 line_endings = "LF" # also "CR", "CRLF", "Native"