Clarify pcall usage and ustring_to_line_array behavior

mods/ITEMS/mcl_signs/API.md

@@ -44,6 +44,7 @@ _UTF-8 strings_, or _u-strings_ for short.
 	* Converts a u-string to string. Used for displaying text in sign formspec
 * `mcl_signs.ustring_to_line_array(ustr)`
 	* Converts a u-string to line-broken list of u-strings aka _a line array_
+	* Behavior on line overflow is controlled by the `mcl_signs_wrap_mode` enum
 * `mcl_signs.generate_line(ustr, ypos)`
 	* Generates a texture string from a u-string for a single line using the
 	  character map

mods/ITEMS/mcl_signs/api.lua

@@ -167,6 +167,7 @@ local function string_to_ustring(str, max_characters)
 
 	local ustr = {}
 
+	-- pcall wrapping to protect against invalid UTF-8
 	local iter = utf8.codes(str)
 	while true do
 		local success, i, code = pcall(iter)