docs: documented 'string_find_next'

.wiki/Kernel-documentation.md

@@ -950,6 +950,9 @@ Returns the amount of chars a string has before it's null-terminator.
 Returns **true** when the strings **_a_** and **_b_** are equal.
 Returns **false** if they aren't equal.
 
+#### `string_find_next(string, chr)` - function (uint32_t)
+Returns the index of the next character that matches **_chr_** in **_string_**.
+
 #### `variadic_format_size(string, args)` - function (uint64_t)
 Returns how long a format string with the given pattern (**_string_**) and **_args_** would be.
 Useful to create a big enough buffer before formatting a string.