style(cfg): apply review items in #385 for gen-config

[ankostis]

Minor changes in the formatting of the generated sample configuration-file,
as reviewed by @ankostis on #385.

The differences in the generated config-file are shown below for this code:

import traitlets.config as trtc
import traitlets as trt

class C(trtc.Configurable):
    a = trt.Enum(['aa', 'BB'], help='1st help line.\n\nOther lines.').tag(config=True)

class A(trtc.Application, C):
    pass

print(A().generate_config_file())

Wihtout this PR:

#------------------------------------------------------------------------------
# A(Application, C) configuration
#------------------------------------------------------------------------------
## This is an application.

## 1st help line.
#  See also C.a
# c.A.a = traitlets.Undefined

WITH this PR:

THIS PR:
##------------------------------------------------------------------------------
## A(Application, C) configuration
##------------------------------------------------------------------------------
## This is an application.

## 1st help line.
#  See also: C.a
#c.A.a = traitlets.Undefined

[minrk]

Thanks. I think only the colon on "See also" is an improvement over what is there now, so can we omit the other two changes?

[minrk]

Let's keep single #, not double here and above.

[ankostis]

Ok

[minrk]

The space between # and c are helpful, let's keep them here.

[ankostis]

Is it possible to keep it?

Otherwise, then the last code line does not differ very little from comments immediately above, and
it needs 2 key strokes to enable the code.

Note, it was like these before #385 (actually i believe it was me that had passed this change in the first place, and had been accepted).

[minrk]

Let's keep the space. It's generally recommended to have a space between the comment symbol and the comment itself. Plus, editors that have a 'comment/uncomment' action generally deal with this in a single keystroke as well.

[ankostis]

OK, kept only See also: change and rebased on latest master.

Regarding the code-lines that now have 1 space between the # char,
it is important that we preserve some difference between the comment-lines.
Currently, the comment-lines start with 2 spaces after #.

So the following expression can collect all code-lines (and possibly some user-edited comments, but that's ok):

sed -n '/^# \w/p'  config.py