improve documentation of evalstr_ex, evalstr

[ThomasBreuer]

• document evalstr_ex (addresses Document GAP.evalstr_ex? #894)
• extend the documentation of evalstr (adresses Change documentation to warn against using GAP.evalstr #897)

Changing evalstr such that GAP's screen output is shown in the Julia session seems to be not possible if the libgap function GAP_EvalString is used. The return value contains strings that describe the screen output plus the return value of each command, and separating the screen output from the string that represents the return value is in general not possible. Note that the pointers to the return values are not useful here because the objects may have changed since they got created, as in the following example.

julia> GAP.evalstr_ex( "x:= []; Add(x, 1);" )
GAP: [ [ true, [ 1 ], false,, "[  ]" ], [ true,, false,, "" ] ]

In the case of functions that have a return value and cause some printing to the screen, we cannot decide which part of the 5th entry in each entry of the result of evalstr_ex belongs to the return value.
(Perhaps libgap's GAP_EvalString should be changed accordingly. The fourth position in the entries of the result is currently unused.)

[codecov]

Codecov Report

Merging #898 (b5fb2da) into master (aac723c) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #898   +/-   ##
=======================================
  Coverage   75.65%   75.65%           
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  Files          51       51           
  Lines        4115     4115           
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  Hits         3113     3113           
  Misses       1002     1002           

Impacted Files	Coverage Δ
src/ccalls.jl	99.16% <ø> (ø)

... and 1 file with indirect coverage changes

[ThomasBreuer]

Some tests got canceled because they were running out of time:

• Julia nightly - ubuntu-latest in "GAP tests",
• Julia 1.8 - macOS-latest in "GAP tests",
• Julia nightly - macOS-latest in "Run tests".