docs: unify doc comments for functions returning bool

[pellared]

Fixes #7063

If also fixes Go Doc comment for SpanID.IsEmpty.

The pattern is based on the way the Go standard library documents functions returning a boolean.
Examples:

• https://pkg.go.dev/strings#Contains
• https://pkg.go.dev/bytes#ContainsFunc
• https://pkg.go.dev/time#Time.Equal
• https://pkg.go.dev/os#IsPathSeparator

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.9%. Comparing base (50868ef) to head (2ff9e4a).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #7064   +/-   ##
=====================================
  Coverage   82.9%   82.9%           
=====================================
  Files        262     262           
  Lines      24460   24460           
=====================================
  Hits       20281   20281           
  Misses      3801    3801           
  Partials     378     378           

Files with missing lines	Coverage Δ
attribute/encoder.go	85.0% <ø> (ø)
attribute/iterator.go	91.7% <ø> (ø)
attribute/key.go	100.0% <ø> (ø)
attribute/kv.go	91.6% <ø> (ø)
attribute/set.go	81.7% <ø> (ø)
bridge/opencensus/internal/span.go	100.0% <ø> (ø)
log/keyvalue.go	94.7% <ø> (ø)
log/logger.go	100.0% <ø> (ø)
sdk/internal/x/x.go	100.0% <ø> (ø)
sdk/metric/instrument.go	97.8% <ø> (ø)
... and 10 more

... and 3 files with indirect coverage changes

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[pellared]

CC @XSAM