Update edit feature implementation details in DG

[xz0127]

Add design considerations for edit feature in DG.
Add activity diagram and sequence diagram for Edit Command in DG.

[codecov-io]

Codecov Report

Merging #135 into master will increase coverage by 0.90%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #135      +/-   ##
============================================
+ Coverage     72.18%   73.08%   +0.90%     
- Complexity      801      849      +48     
============================================
  Files           113      119       +6     
  Lines          2520     2642     +122     
  Branches        312      322      +10     
============================================
+ Hits           1819     1931     +112     
- Misses          615      621       +6     
- Partials         86       90       +4     

Impacted Files	Coverage Δ	Complexity Δ
.../java/seedu/address/logic/parser/NuudleParser.java	61.53% <0.00%> (-5.13%)	12.00% <0.00%> (ø%)
.../java/seedu/address/logic/commands/AddCommand.java	100.00% <0.00%> (ø)	8.00% <0.00%> (ø%)
...java/seedu/address/logic/commands/DoneCommand.java	100.00% <0.00%> (ø)	11.00% <0.00%> (ø%)
...ava/seedu/address/logic/commands/ClearCommand.java	100.00% <0.00%> (ø)	2.00% <0.00%> (ø%)
...va/seedu/address/logic/commands/CancelCommand.java	100.00% <0.00%> (ø)	10.00% <0.00%> (ø%)
...va/seedu/address/logic/commands/ChangeCommand.java	0.00% <0.00%> (ø)	0.00% <0.00%> (ø%)
...va/seedu/address/logic/commands/DeleteCommand.java	100.00% <0.00%> (ø)	7.00% <0.00%> (ø%)
...ess/model/exceptions/NoUndoableStateException.java	100.00% <0.00%> (ø)	1.00% <0.00%> (?%)
...ess/model/exceptions/NoRedoableStateException.java	100.00% <0.00%> (ø)	1.00% <0.00%> (?%)
...java/seedu/address/logic/commands/UndoCommand.java	81.81% <0.00%> (ø)	3.00% <0.00%> (?%)
... and 7 more

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[JinHao-L]

LGTM 👍
The text in the diagram looks a bit small, but its not really an issue. (We can clean the documentation in milestone 1.4