Correct default lines of scrollback in the docs.

The default was increased in fd266a3 (just before 0.63).
Refer to X11 'graphical applications'.

For people who know they need graphical applications but don't know what
X11 is.
Clarify that port forwarding is of TCP connections.

(Rather than just the generic 'network connections'.)
Rearrange SSH bug docs to match the GUI.

No change to the text.
Tweak SSH protocol version refusal messages.

"required by user" will grate if the user did not configure the
behaviour (and I'm about to change the default to `2 only').
Tweak key file config docs re Pageant.

Clarify that it's not necessary to faff around with explicit key
configuration if you're using Pageant.
Don't try SSH-1 by default.
Cross-reference 'logical host' and host key config docs.
Discourage SSH-1 in documentation.
Merge misc docs improvements from branch 'pre-0.64'.
Merge SSH-2-only default from 'pre-0.64'.