Bug#919392: Warn on man page there is no way to replace headers, only add to them

[積丹尼 Dan Jacobson]

Package: w3m
Version: 0.5.3-37
Severity: wishlist
File: /usr/share/man/man1/w3m.1.gz

       -header string
              append string to the HTTP(S) request. Expected to match the 
header syntax Variable: Value

Add: Note there is no way to replace headers: -header "User-Agent: xyz"
will simply add a second User-Agent header.