f dist approx for joint test

[yiqunchen]

added f dist approx for joint test; fixed the r manual error

[codecov-commenter]

Codecov Report

Merging #115 (54dddc1) into main (c77fe1f) will decrease coverage by 0.45%. The diff coverage is 40.00%.

@@            Coverage Diff             @@
##             main     #115      +/-   ##
==========================================
- Coverage   74.37%   73.92%   -0.46%     
==========================================
  Files          21       21              
  Lines        3333     3367      +34     
==========================================
+ Hits         2479     2489      +10     
- Misses        854      878      +24     

Impacted Files	Coverage Δ
R/lincom_utils.R	0.00% <0.00%> (ø)
R/lincom.R	79.38% <30.76%> (-17.99%)	:arrow_down:
R/regress.R	95.84% <62.50%> (-0.68%)	:arrow_down:
R/descrip.R	95.21% <75.00%> (-0.44%)	:arrow_down:

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