doc/api: Fix return values for callback_read in config_new_section

doc/api: Fix return values for callback_read in config_new_section

These return values were wrong in the description and C example. As can
be seen on lines 2835 and 2873-2903 of src/core/wee-config-file.c the
callback_read function should return the same as the function
config_file_option_set plus the value
WEECHAT_CONFIG_OPTION_SET_OPTION_NOT_FOUND (which is also the same as
the possible return values of config_file_option_set_with_string).

The Python example was already correct and the C example was already
correct in the other languages apart from English.

These errors were introduced in commit 02e2b21d3 and commit 5210ff1ae.