common/libutil: Fix base64 API documentation

[chu11]

base64_encode_block() and base64_decode_block() require dstlen
to be passed in, but the header files say they are optional. Adjust
documentation appropriately.

[chu11]

FWIW, I went back and forth on "fix documentation" or "fix function to do what it says". B/c many other functions require dstlen, I just went with that.

[garlick]

Oh yep that's true. @dun might want to grab that too, since we raided munge for that code.

[coveralls]

Coverage increased (+0.1%) to 78.022% when pulling cbe0526 on chu11:base64_documentation into 71b40ff on flux-framework:master.

[codecov-io]

Codecov Report

Merging #1138 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1138      +/-   ##
==========================================
+ Coverage   77.57%   77.61%   +0.03%     
==========================================
  Files         157      157              
  Lines       28679    28678       -1     
==========================================
+ Hits        22247    22257      +10     
+ Misses       6432     6421      -11

Impacted Files	Coverage Δ
src/common/libkvs/kvs_watch.c	86.47% <0%> (-0.97%)	⬇️
src/common/libflux/attr.c	87.67% <0%> (-0.92%)	⬇️
src/broker/overlay.c	71.67% <0%> (-0.35%)	⬇️
src/common/libflux/message.c	80.7% <0%> (-0.24%)	⬇️
src/bindings/lua/flux-lua.c	81.67% <0%> (-0.1%)	⬇️
src/common/libkvs/kvs.c	65.33% <0%> (+0.25%)	⬆️
src/modules/kvs/kvs.c	63.45% <0%> (+0.26%)	⬆️
src/common/libcompat/rpc.c	94.57% <0%> (+0.72%)	⬆️
src/common/libflux/response.c	84.55% <0%> (+0.81%)	⬆️
src/broker/modservice.c	80.58% <0%> (+0.97%)	⬆️
... and 2 more

[dun]

You're mixing verb tenses within a sentence. It should read "and sets [dstlen] to the number of bytes written."

[chu11]

fixed and re-pushed

[coveralls]

Coverage increased (+0.03%) to 77.96% when pulling e5bf5aa on chu11:base64_documentation into 9e5af83 on flux-framework:master.

[garlick]

Thanks guys.