doc: lei-overview: implicit stdin, correct Inline::C notes
Implicit stdin based on standard input being a pipe or regular
file is here to stay, so save users the trouble of typing '-'
or '--stdin'.
Inline::C is required as of commit
1d6e1f9a6a66 (lei: require
Socket::MsgHdr or Inline::C, drop oneshot, 2021-05-26); but
Socket::MsgHdr still gives a noticeable improvement in bash
completion speed.
Also, spell-out "MESSAGE-ID" since "MID" is actually not a
common abbreviation ("MSGID" is used by RFC 3977 and several
other RFCs, I recall).