Expose the Client::send method, adding a brief doccomment

[Alch-Emi]

This was briefly discussed in the Matrix room, but I believe that exposing the Client::send at least until the 1.0 release may be beneficial, as the ruma api is extensive and fairly easy to use, thus greatly extending the functionality of this library until more wrapper methods are complete. The doccomment used for this method indicates that it is not recommended to use this method while a wrapper method exists for the same purpose, and warns that this method will not update client state.

[codecov-commenter]

Codecov Report

Merging #40 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #40   +/-   ##
=======================================
  Coverage   75.39%   75.39%           
=======================================
  Files          23       23           
  Lines        4386     4386           
=======================================
  Hits         3307     3307           
  Misses       1079     1079           

Impacted Files	Coverage Δ
matrix_sdk/src/client.rs	78.19% <100.00%> (ø)

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[poljar]

Could we get an example in the docs there. I think most people that won't look at the rust-sdk code will be a bit confused.