doc: improve environment variables documentation

[marcinslusarz]

This change is 

[krzycz]

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Reviewed 1 of 1 files at r1. Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[codecov-io]

Codecov Report

Merging #316 into master will decrease coverage by <.01%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #316      +/-   ##
==========================================
- Coverage    77.5%   77.49%   -0.01%     
==========================================
  Files          90       90              
  Lines       13811    13812       +1     
  Branches     1994     1994              
==========================================
  Hits        10704    10704              
  Misses       2383     2383              
- Partials      724      725       +1

Impacted Files	Coverage Δ
src/libpmemfile-posix/read.c	89.79% <0%> (-0.55%)	:arrow_down:
src/libpmemfile-posix/file.c	66.39% <0%> (-0.37%)	:arrow_down:
src/libpmemfile-posix/data.c	91.92% <0%> (-0.36%)	:arrow_down:
src/libpmemfile-posix/dir.c	82.01% <0%> (+0.2%)	:arrow_up:
src/libpmemfile-posix/rename.c	91.12% <0%> (+0.4%)	:arrow_up:

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[sarahjelinek]

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Review status: :shipit: all files reviewed at latest revision, all discussions resolved, all commit checks successful.

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Comments from Reviewable