Update docs to add notes about BEIR uniCOIL and HC4 regressions

[lintool]

% python src/main/python/beir/gather_beir_results.py 
nDCG@10                  flat    multi   UCx     SPLADE
                         ------  ------  ------  ------
trec-covid               0.5947  0.6559  0.6403  0.7109
bioasq                   0.5225  0.4646  0.4771  0.5035
nfcorpus                 0.3218  0.3254  0.3325  0.3454
nq                       0.3055  0.3285  0.4249  0.5442
hotpotqa                 0.6330  0.6027  0.6671  0.6860
fiqa                     0.2361  0.2361  0.2888  0.3514
signal1m                 0.3304  0.3304  0.2750  0.2957
trec-news                0.3952  0.3977  0.3743  0.3936
robust04                 0.4070  0.4070  0.4030  0.4581
arguana                  0.3970  0.4142  0.3959  0.5210
webis-touche2020         0.4422  0.3673  0.2976  0.2435
cqadupstack-android      0.3801  0.3709  0.3563  0.3954
cqadupstack-english      0.3453  0.3321  0.3635  0.4026
cqadupstack-gaming       0.4822  0.4418  0.4459  0.5061
cqadupstack-gis          0.2901  0.2904  0.2713  0.3223
cqadupstack-mathematica  0.2015  0.2046  0.2106  0.2423
cqadupstack-physics      0.3214  0.3248  0.3121  0.3668
cqadupstack-programmers  0.2802  0.2963  0.2967  0.3412
cqadupstack-stats        0.2711  0.2790  0.2860  0.3142
cqadupstack-tex          0.2244  0.2086  0.2279  0.2575
cqadupstack-unix         0.2749  0.2788  0.2826  0.3292
cqadupstack-webmasters   0.3059  0.3008  0.2974  0.3343
cqadupstack-wordpress    0.2483  0.2562  0.2573  0.2839
quora                    0.7886  0.7886  0.6625  0.8136
dbpedia-entity           0.3180  0.3128  0.3383  0.4416
scidocs                  0.1490  0.1581  0.1439  0.1590
fever                    0.6513  0.7530  0.8118  0.7962
climate-fever            0.1651  0.2129  0.1824  0.2276
scifact                  0.6789  0.6647  0.6856  0.6992
                         ------  ------  ------  ------
                         0.4244  0.4288  0.4279  0.4740

[codecov-commenter]

Codecov Report

Merging #1915 (c0717df) into master (f592832) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1915   +/-   ##
=========================================
  Coverage     58.67%   58.67%           
  Complexity     1079     1079           
=========================================
  Files           180      180           
  Lines         10420    10420           
  Branches       1429     1429           
=========================================
  Hits           6114     6114           
  Misses         3786     3786           
  Partials        520      520           

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