Add clear -f command to User Guide and fix typos

[jamesyeap]

Fixes #223
Fixes #232
Fixes #212

This pull-request adds

• a description of the clear -f command to the User Guide
• edits to the findstudents and findlessonssections of the User Guide
• a draft of the PPP for James

[codecov-commenter]

Codecov Report

Merging #236 (64edb4b) into master (c6b80c7) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #236   +/-   ##
=========================================
  Coverage     48.17%   48.17%           
  Complexity      594      594           
=========================================
  Files           125      125           
  Lines          2788     2788           
  Branches        305      305           
=========================================
  Hits           1343     1343           
  Misses         1352     1352           
  Partials         93       93           

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[nnmoq]

LGTM! Should split this into multiple PRs next time.