1) Following command searches the use of an 'a' article used before a word that starts with 'a' letter.

grep -riI " a a" --include="*.md" .

and in master branch it finds:

a arithmetical 

./manual/config/items/preprocessing/jsonpath_functionality.md:

Filter expression is a arithmetical expression in infix notation.

which is wrong.

2) Similarly, the following search:

grep -rIi " an u" --include="*.md" .

finds:

an URL

./manual/api/reference/script/create.md
which is also wrong.
3.1) Command

grep -riI " an [bcdfghjklmnpqrstvwxz]"  --include="*.md" .

will find several places like:

an sql

./manual/config/items/itemtypes/zabbix_agent/zabbix_agent2.md

must be equal to a name of an sql file

3.2)

an token

./manual/api/reference/token/get.md

Retrieve an token

3.3) and 3.4)

an line, an geometric

./manual/api/reference/map/object.md

an line displayed
an geometric shape