perlfunc: document relationship between wait() and waitpid() #470

Job	Run time
Sanity: Linux -Dusethreads	10m 16s
involved_authors_debug	0s
install	10m 19s
linux (-Uusethreads)	9m 23s
linux (-Duseshrplib -Dusequadmath -Dusecbacktrace -Dusethreads)	11m 13s
cygwin	34m 2s
Windows mingw64	32m 51s
linux (-Duserelocatableinc)	9m 41s
linux i386/ubuntu (-Dusedevel)	15m 38s
linux (-Dcc='clang')	9m 51s
linux (-Dcc='g++')	9m 34s
ASAN (-Accflags=-DPURIFY)	21m 1s
linux (-Accflags=-DSILENT_NO_TAINT_SUPPORT)	9m 20s
dist-modules (latest)	2m 41s
ASAN (-DDEBUGGING)	22m 18s
linux (-Accflags=-DNO_TAINT_SUPPORT)	9m 12s
dist-modules (5.24)	2m 41s
ASAN (-Dusethreads)	22m 30s
linux (-DDEBUGGING -Accflags=-DPERL_RC_STACK)	9m 46s
dist-modules (5.10)	2m 23s
dist-modules (5.8)	2m 20s
Test dist/ modules on MacOS system perl	5m 15s
minitest	3m 32s
PERL_UNICODE (-Uusethreads)	8m 15s
macOS (big sur) xcode 13 (-Uusethreads)	14m 30s
PERL_UNICODE (-Dusethreads -Accflags=-DPURIFY -Dcc='gcc -fsanitize=address' -Dld='gcc -fsanitize=...	23m 18s
Windows msvc142	51m 48s
macOS (big sur) xcode 13 (-Duseithreads -Duseshrplib)	16m 31s
	6h 20m 9s

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