Update rc-files documentation

[eval]

Bringing docs in line with what (and in what order) rc files are loaded:

• $HOME/.config/irb/irbrc is only considered when XDG_CONFIG_HOME is not set.
• loading of irb.rc was removed in 11d03a6ff771e17c4b7808dd45e7329cca7f68d3

Verified order with files containing puts "Loading #{__FILE__}":

❯ env IRBRC=tmp/irbrc_env_var XDG_CONFIG_HOME=tmp/xdg_config_home irb
Loading tmp/irbrc_env_var
Loading tmp/xdg_config_home/irb/irbrc
Loading /Users/gert/.irbrc
Loading /Users/gert/projects/uri-smtp/uri-smtp/.irbrc
Loading /Users/gert/projects/uri-smtp/uri-smtp/_irbrc
Loading /Users/gert/projects/uri-smtp/uri-smtp/$irbrc
irb(main):001>