Add docs for app-manifest-cache-count

[ffjlabo]

What this PR does / why we need it:

as title

Which issue(s) this PR fixes:

Follow #5108

Does this PR introduce a user-facing change?:

• How are users affected by this change:
• Is this breaking change:
• How to migrate (if breaking change):

[khanhtc1202]

Nice catch

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 22.80%. Comparing base (378ab0f) to head (a58dd2c).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5131      +/-   ##
==========================================
+ Coverage   22.79%   22.80%   +0.01%     
==========================================
  Files         412      412              
  Lines       43827    43827              
==========================================
+ Hits         9989     9996       +7     
+ Misses      33050    33044       -6     
+ Partials      788      787       -1     

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