Eeschema User Manual description of "Template Field Names" (now "Default Fields") should be improved (lp:#1575100)

[Original report](https://bugs.launchpad.net/bugs/1575100) created by **Robin Whittle (rw-p)**
```
The Eeschema User Manual has a brief, uninformative and I think incorrect
explanation of the Eeschema preference items "Template Field Names" as they
are known in 4.0.2-stable.  In the latest nightly (2016-04-23 BZR 6710, Git
c1f0ab9) this is known as "Default Fields", which I think is a better name.

The current description is:

     "You can define custom fields that will exist by default in each
component (even if left empty)."

This is an inadequate description of a very useful function, and it is
incorrect in that these fields do not exist in the schematic components or
their representation in schematic libraries.  They are presented as fields
in the edit dialogues but they are only written to the component in the
schematic or library if "OK" is chosen to close the edit dialogue.

I explored the behavior of this system with 4.0.2-stable at:
http://www.firstpr.com.au/kicad/#tfn .  I assume the behavior is the same
in the latest nightly.  The latest nightly has a new and improved user
interface which avoids confusion due to text boxes and an "Add" button,
which I think is the cause of a bug report
https://bugs.launchpad.net/kicad/+bug/1389272 which I think could now be
marked as resolved.

I suspect that the new user interface does not require any explanation.
Here is some suggested text to replace the current single sentence.
Ideally someone would check the validity of this and revise it to fit the
style of the manual.

- - -

Default Fields are a mechanism for easily introducing additional fields
into components in schematics or component definitions in schematic
libraries.

Each Default Field consists of a field name, a default value for this field
- which can be empty - and a flag which controls whether the value of this
field will be visible in the schematic editing screen and the printed
schematic.

These configuration items are part of the Eeschema configuration and are
not part of any schematic sheet or project.

The one or more Default Fields specified here do not exist in any
components in schematics or in  schematic libraries until each such
component is edited and the "OK" button is pressed in the edit dialogue
box.  Then, the one or more Default Fields are added to the already
existing fields for this component, starting at field 4, with any
previously existing fields 4 and above being renumbered accordingly.
Changes can be made to the value and/or visibility flag during this initial
edit session or at any subsequent time.

Later changes to the default value of a Default Field will not affect the
value of fields which have already been added to components.  Once one or
more fields have been added to a component, each also has parameters for
position, size and orientation of the text, when displayed in the
schematic.

- - -

The ability to add one or more new fields into components, per component or
at the library level, is powerful and important.  It is the basis for
several attempts at creating greatly improved BOMs and the like, such as
those described at: https://rheingoldheavy.com/category/education/kicad/  .

I think the explanation could be enhanced at the start or end by some
mention of the use of these fields in creating sophisticated schematic-
based data systems as the basis for generating BOMs.
```

Story points: 6.0