Fix error with validating booking period and update user guide

[AnnabelTing]

Close #239, #220

[codecov[bot]]

Codecov Report

Merging #256 (554aa94) into master (9f7c36d) will increase coverage by 0.05%. The diff coverage is 50.00%.

@@             Coverage Diff              @@
##             master     #256      +/-   ##
============================================
+ Coverage     70.76%   70.82%   +0.05%     
  Complexity      565      565              
============================================
  Files            94       94              
  Lines          1878     1878              
  Branches        211      210       -1     
============================================
+ Hits           1329     1330       +1     
  Misses          488      488              
+ Partials         61       60       -1     

Files	Coverage Δ
...in/java/seedu/address/logic/parser/ParserUtil.java	94.00% <100.00%> (ø)
...rc/main/java/seedu/address/model/booking/Room.java	94.11% <ø> (ø)
...rc/main/java/seedu/address/model/person/Email.java	100.00% <ø> (ø)
...rc/main/java/seedu/address/model/person/Phone.java	100.00% <ø> (ø)
...ava/seedu/address/model/booking/BookingPeriod.java	86.30% <33.33%> (+1.36%)	:arrow_up:

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[RB9823]

LGTM