docs: use default timeout for restore operation

[olavloite]

The restore operation should use the default timeout of a restore operation. Although that timeout is long, the tests for the samples are only executed as part of the nightly build, which means that a long-running restore will not slow down PR builds.

Fixes #1019

[codecov]

Codecov Report

Merging #1109 (a558b26) into master (37ca990) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1109      +/-   ##
============================================
- Coverage     84.99%   84.96%   -0.03%     
+ Complexity     2727     2723       -4     
============================================
  Files           156      156              
  Lines         14410    14410              
  Branches       1379     1379              
============================================
- Hits          12248    12244       -4     
- Misses         1593     1597       +4     
  Partials        569      569              

Impacted Files	Coverage Δ	Complexity Δ
.../google/cloud/spanner/AbstractLazyInitializer.java	92.85% <0.00%> (-7.15%)	4.00% <0.00%> (-1.00%)
...ud/spanner/SessionPoolAsyncTransactionManager.java	85.03% <0.00%> (-1.58%)	13.00% <0.00%> (-2.00%)
...m/google/cloud/spanner/connection/SpannerPool.java	87.16% <0.00%> (-0.54%)	33.00% <0.00%> (ø%)
...ain/java/com/google/cloud/spanner/SessionPool.java	89.01% <0.00%> (ø)	74.00% <0.00%> (ø%)
...a/com/google/cloud/spanner/SessionPoolOptions.java	69.53% <0.00%> (ø)	17.00% <0.00%> (-1.00%)

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